The surface activity coefficients of ideal mixtures adsorbed on the heterogeneous surfaceshave been calculated systematically by using the two-site heterogeneous surface model. The calculatedrcsults show that the apparent surface activity coefficients are alwnys smaller than 1, i.e., negativedeviation from Raoult’s law, which were always found experimentally. On the other hand, the calculatedsurface activity coefficients curves are always nearly symmetric if the total surface coverage is closeto 1. This situation is comparable wth those ofadsorption from solutions. Therefore, the asymmetriccurves for the surface activity coefficients may not be due to the heterogeneous of the surface, butdue to the nonideal molecular interactions between the unlike components of the adsorbed mixture. The surface activity coefficients of ideal mixtures adsorbed on the heterogeneous surfaces have been calculated systematically by using the two-site heterogeneous surface model. The calculated results show that the apparent surface activity coefficients are alwnys smaller than 1, ie, negativedeviation from Raoult’s law, which were always found experimentally. On the other hand, the calculatedsurface activity coefficient curves are always nearly symmetric if the total surface coverage is closeto 1. This situation is comparable wth those ofadsorption from solutions. Thus, the asymmetric curves for the surface activity coefficients may not be due to the heterogeneous of the surface, butdue to the nonideal molecular interactions between the unlike components of the adsorbed mixture.