A statistical mechanical theory of monolayer adsorption on to a plane charged surface from a mixture of two polar solvents is described. The region of the mixed monolayer containing adsorbed molecules with permanent and induced dipoles is treated as a vacuum situated between two continuous dielectric media. The ratio of areas occupied by the two components equals 2 and each adsorbed molecule is assigned two dipole orientation states in the monolayer. Preferential dipole orientation at the point of zero charge is achieved by postulating a residual energy. Because of the strong lateral dipole-dipole interactions and the molecular polarisability, the mean dipole moment per unit area of monolayer is small enought to give an acceptable potential drop across the monolayer. By introducing a replacement or exchange condition, the theoretiacal individual and composite adsorption isotherms are determined at different surface charges.