An improved model for the equation of state for Langmuir monolayers proposed in J. Phys. Chem. B 1999, 103, 145 is introduced for the case where two or more phase transitions occur in the monolayer. The model allows the theoretical description of the phase transition between the two condensed phases under the realistic precondition that the phase transition is accompanied by the change of molecular compressibility parameters in the condensed phases. N-Alkyl-beta-hydroxy-propionic acid amide monolayers undergo two phase transitions during the compression at low temperature, demonstrated experimentally both by the surface pressure-area (Pi-A) isotherm and the results of grazing incidence X-ray diffraction (GIXD) for the C(13)H(27) and C(14)H(29) alkyl chain lengths. The theoretical Pi-A isotherms obtained by the new model agree well with the experimental Pi-A isotherms and with the data obtained from the GIXD experiments.