The dynamics of a mixed monolayer consisting of a soluble tailored amphiphile N-decyl-gamma-hydroxybutyric acid amide (C10NH) and its insoluble condensing homologue N-tetradecyl-gamma-hydroxybutyric acid amide (C14NH) are experimentally and theoretically studied. Single-component and mixed monolayers of the soluble C10NH and the insoluble C14NH are studied experimentally, and the results are compared with theoretical predictions. For some theoretical models. equations of state for mixed monolayers of an insoluble and a soluble homologue and the adsorption isotherm equation for soluble amphiphile are derived. Assuming diffusion kinetics for the desorption of the soluble component during the compression of the mixed monolayer, a new equation for the surface concentration (adsorption) of the soluble amphiphile allows one to analyze this value with dependence on the conditions of the monolayer compression, concentration, and other parameters. The influence of the nonideality of the mixture on the shift of the area value per molecule of the insoluble amphiphile, which corresponds to the commencement of the two-dimensional condensation in the monolayer, is theoretically treated. The theoretical predictions are in a satisfactory agreement with the experiments. It is shown that the account for the nonideality in the mixture of amphiphiles important for the qualitative and quantitative analysis of the formation of domains in the mixed monolayer.