1. In the adsorption of a mixture of two substances in accordance with Langmuir's equation on a homogeneous surface, the adsorption phase behaves as an ideal two-dimensional solution obeying Raoult's law. 2. In contrast to two-component nonideal three-dimensional solutions, for which deviations from Raoult's law for the two components are either simultaneously positive or simultaneously negative, for adsorption solutions on the surface of a solid with the total adsorption isostere linear, these deviations are negative for the component adsorbed most and positive for the component adsorbed least. 3. Calculations for a particular case show that this anomaly in the properties of surface solutions is caused by the heterogeneity of the surface, and the deviations from Raoult's law characterize this heterogeneity rather than the properties of the surface solution. 4. An examination was made of the possible effect of interaction forces between adsorbed molecules on phase diagrams of surface solutions and the form of total adsorption isosteres.