This thesis describes the results of ab initio molecular orbital calculations employing a minimal basis of Gaussian orbitals on mono and disubstituted benzenes containing all possible combinations of the following substituents : H, F, CN, OH, CHO, CH 3) NH2 and NO2. The work is presented in three parts. The first outlines molecular orbital theory and the mathematical formulation of the one-electron properties which are discussed at a later point in the thesis. Part II describes the computational procedures employed. The computer programs which were used during the course of the work are discussed in some detail as are the Gaussian basis sets which were employed in the calculations. In particular, the scaling procedure and its effect on the calculated molecular properties is examined and a new method of upgrading the minimal basis to a double zeta one is also discussed. These double zeta basis sets were used in some of the calculations on the monosubstituted compounds. The third part of the thesis contains the discussion of the calculations. The close relationship between the wave functions of the benzene orbitals and those of the monosubstituted compounds is examined using contour diagrams of the molecular electron densities generated by a program specially written for the purpose. The text of this program and detailed operating instructions are included in an appendix. Wherever possible the calculated molecular properties are compared with empirical values. In particular, the calculations are used to assign the photoelectron spectra of many of the substituted benzenes.