Expressions for the second and fourth sum rules of relative and cross velocity correlation functions are derived for two component systems. These sum rules and Mori's memory function formalism have been used to study the mutual, self-and distinct diffusion in equimolar Ar-Kr mixture and molten salts. It is found that distinct diffusion, which is a measure of effect of cross correlations representing momentum spread to neighbouring particles, is positive for Ar-Kr mixture and is negative for molten salts. These findings are in agreement with molecular dynamics predictions. The positive value of distinct diffusion was not accounted for by Darken's and common force models.