Compositional variation in a vertical rectangular porous medium containing a two-component single phase fluid in the presence of a prescribed linear temperature field is considered. We investigate by using a finite volume method, the effects of convection, thermal diffusion, and pressure diffusion on the compositional variation. Calculations show that, for high permeabilities, convective mass transfer overrides both thermal and pressure diffusion; it is the main phenomenon affecting compositional variation within the porous medium. In this case, the permeability increase makes the composition variation less pronounced. For low permeabilities, compositional variation is mainly affected by the ratio (-C-T T-z)/(C-P P-z) where C-P C-T, P-z, and T-z are the pressure and thermal diffusion coefficients and the vertical pressure and thermal gradients, respectively. When this ratio is >1, thermal diffusion is the main phenomenon affecting compositional variation and the horizontal composition gradient reaches a maximum at some permeability. For (-(CTz)-T-T)/((CPz)-P-P) <1, compositional variation is mostly affected by pressure diffusion and the horizontal compositional gradient decreases monotonically with permeability increase.