A variety of smectic phase diagrams involving smectic-isotropic and smectic-nematic-isotropic transitions have been calculated based on a combination of Flory-Huggins (FH) theory for isotropic mixing and Maier-Saupe-McMillan (MSM) theory for smectic-A ordering of liquid crystals (LC). To describe the mesophase transitions, two nematic order parameters and two smectic order parameters have been coupled through the normalized orientation distribution and partition functions. Flory-Huggins interaction parameter (chi) for isotropic mixing and the coupling term involving the nematic interaction parameter (nu) and the McMillan smectic interaction parameter (alpha) for phase transitions of liquid crystals have been incorporated in the calculation. The predictive capability of the present combined FH/MSM model for determining the coexistence regions of a binary smectic-A mixture has been demonstrated by critically testing with a reported smectic phase diagram. (C) 1997 American Institute of Physics. [S0021-9606(97)51441-3].