The problem of barium sulphate scale in oil field operations has been studied from the analytical point of view. The literature on barium sulphate solubility in different media was reviewed and an assessment of the problem made. A new method was developed for the determination of barium in the presence of high concentrations of other alkaline earth and sodium ions. DCTA was used for differential complexing at appropriate pH and sulphonazo III was used as the reagent. Barium sulphate solubility was determined experimentally at 25 - 95 C and pressures ranging from atmospheric, to 9000 psig, in water and in various concentrations of sodium chloride solutions to simulate oil reservoir conditions. The trends in solubility were explained on the basis of complex formation with the sodium and chloride ions present. A critical review was made of the available stability constant values for the complexes concerned and some of the values in the literature were recalculated and also experimentally determined. A mechanistic theory was developed to account for the combined effects of temperature, pressure and salt concentration. A rapid method for the initial screening of chemical scale inhibitors was also developed and discussed. The significance of the work for dealing with field problems was appraised and future developments suggested.