The effects of mutual diffusion between O and N2 are examined for diurnal and semidiurnal tidal oscillations in the thermosphere. A binary gas extension of a previous F plane equivalent gravity wave formalism is developed and utilized to study this effect. Compared with calculations that omit O-N2 mutual diffusion, diffusive interaction affects the temperatures and horizontal velocities associated with the diurnal thermospheric tide excited in sity by less than 10% with negligible shifts in phase, and by roughly 20--40% the semidiurnal tide excited in sity, and the 2, 2) and (2, 4) modes propagating from the mesosphere, accompanied by 1- to 4-hour shifts in phase. These results affect the interpretation of incoherent scatter measurements of thermospheric tides and the construction of synthesized tidal models that are calibrated by the measurements and suggest that the ineauality of the vertical transport velocities of O and N2 be accounted for in three-dimensional numerical models of semidiurnal thermospheric tides.