The equation of state for Langmuir monolayers proposed earlier (Fainerman, V. B.; Vollhardt, D. J. Phys. Chem. B 1999, 103, 145.) is generalized for the case that one, two or more phase transitions in the monolayer take place, provided that the two-dimensional compressibility undergoes a jump at the phase transition, whereas within any condensed phase of the monolayer this compressibility is proportional to the surface pressure. N-Tetradecyl-beta-hydroxy-propionic acid amide (THPAA) monolayers undergo two phase transitions during the compression, demonstrated experimentally both by the cusps of the surface pressure-area (Pi-A) isotherm and the results of grazing incidence X-ray diffraction (GIXD). The theoretical Pi-A isotherm agrees satisfactorily with the experimental data if the THPAA molecular characteristics obtained from independent GIXD experiments are taken as model parameters.