Knowledge of the phase behaviour of mixtures of oil with carbon dioxide and water is essential for reservoir engineering, especially in the processes of enhanced oil recovery and geological storage of carbon dioxide. Both processes require versatile tools able to describe the global phase behaviour at reservoir conditions, which may include the critical region of the mixtures involved. For a comprehensive understanding however the study of simpler systems needs to be completed. In this work two ternary systems have been studied as models for (oil + carbon dioxide + water) mixtures. The first one consists of (n-decane + carbon dioxide + water); the second is a mixture of (propane + carbon dioxide + water). To measure phase equilibria at representative reservoir conditions, a new analytical apparatus has been designed with maximum operating temperature and pressure of 423K and 45MPa, respectively. The equipment relies on recirculation of two coexisting phases using a two-channel magnetically-operated micro-pump designed during this work, sampling and on-line compositional analysis by gas chromatography. The apparatus has been validated by comparison with published isothermal vapour-liquid equilibrium data for the binary system (n-decane + carbon dioxide). New experimental data have been measured for the systems (ndecane + carbon dioxide + water) and (propane + carbon dioxide + water) under conditions of three-phase equilibria. Data for the three coexisting phases in the mixture of (n-decane + carbon dioxide + water) have been obtained on five isotherms at temperatures from (323 to 413)K and at pressures up to the point at which two of the phases become critical. Similarly, for the mixture (propane + carbon dioxide + water), data for the three coexisting phases on four isotherms at temperatures from (311 to 353)K and pressures up to the same point are reported. The experimental work has been complemented here with a theoretical effort in which models for these molecules are developed within the framework of the statistical associating fluid theory for potentials of variable range (SAFT-VR). The phase behaviour of the three binary subsystems has been calculated using this theory and, where applicable, a modification of the Hudson and McCoubrey combining rules has been used to treat the systems predictively. The experimental data obtained for the ternary mixture are compared to the predictions of the theory. Furthermore, a detailed analysis of the ternary mixture is carried out based on comparison with available data for the constituent binary subsystems. In this way, the observed effects on the solubility when the third component is added are analysed. An accurate prediction of phase behaviour at conditions far and close to criticality cannot be accomplished by mean-field based theories, such as the SAFT-VR equation of state, that do not incorporate long-range density fluctuations. A treatment based on renormalisation-group (RG) theory as developed by White and co-workers has proven very successful in improving the predictions of the critical region with different equations of state. The basis of the method is an iterative procedure to account for contributions to the free energy of density fluctuations of increasing wavelengths. The RG method has been combined with a number of versions of the statistical associating fluid theory (SAFT), by implementing White’s earliest ideas with the improvements of Prausnitz and co-workers. Typically, this treatment involves two adjustable parameters: a cut-off wavelength L for density fluctuations and an average gradient of the wavelet function Φ. In this work, the SAFT-VR equation of state has been extended with a similar crossover treatment which however follows closer the most recent improvements introduced by White. The interpretation of White’s latter developments allows one to establish a straightforward method which enables Φ to be evaluated; only the cut-off wavelength L needs then be adjusted. The approach used here begins with an initial free energy incorporating only contributions from short-wavelength fluctuations, which are treated locally. The contribution from long-wavelength fluctuations is incorporated through an iterative procedure based on attractive interactions which incorporate the structure of the fluid following the ideas of perturbation theories and using a mapping that allows integration of the radial distribution function. Good agreement close and far from the critical region is obtained using a unique fitted parameter L that can be easily related to the range of the potential. In this way the thermodynamic properties of a square-well (SW) fluid are given by the same number of independent intermolecular model parameters as in the classical equation. Far from the critical region the approach provides the correct limiting behaviour reducing to the classical equation (SAFT-VR). In the critical region the β critical exponent is calculated and is found to take values close to the universal value. In SAFT-VR the free energy of an associating chain fluid is obtained following the thermodynamic perturbation theory of Wertheim from knowledge of the free energy and radial distribution function of a reference monomer fluid. By determining L for SW fluids of varying well width a unique equation of state is obtained for chain and associating systems without further adjustment of critical parameters. Computer simulation data of the phase behaviour of chain and associating SW fluids are used to test the accuracy of the new equation. Furthermore the treatment is here extended to model pure fluids and results are presented for a number of alkanes, carbon dioxide and water.