The monolayer features of two very similar amphiphiles, N-tridecyl-beta-hydroxypropionic acid amide (THPA) and N-(beta-hydroxyethyl) tridecanoic acid amide (HETA) and their 1:1 mixture are studied. The behavior of the mixed monolayer is of special interest as already the pure components reveal large differences of the surface pressure-area (pi-A) isotherms, in particular, because of the second phase transition between two condensed phases at low temperatures in the THPA monolayer. This second phase transition occurs in the mixed monolayer. as well. The thermodynamic characteristics of the mixed HETA/THPA monolayers are compared with those of the pure components. The thermodynamic analysis is performed on the basis of the equations of state for the monolayer in the fluid (G, LE)/condensed (LC) transition region (A < A(c))with bimodal distribution (large clusters and monomers) and in the fluid (G, LE) state (A >= A(c))considering the contribution of the entropy nonideality, caused by the mixing of monomers and clusters. Good agreement between the experimental pi-A isotherms and the theoretical results are obtained. The results of the thermodynarnic analysis allow conclusions on the specific phase properties of the mixed HETA/THPA monolayers. Finally the additive model applied for the theoretical description at higher temperatures provides good agreement with the experimental results at temperatures of >= 20 degrees C.