A comparative analysis of theoretical models, which describes the displacement of the two-dimensional phase equilibrium in monolayers at the air/water interface, is performed. The thermodynamic characteristics for the first-order phase transition are calculated on the basis of various theoretical models. The calculations are performed for the experimental surface pressure-area (Pi-a) isotherms of three branched chain phospholipids (n-16PEs) with different side chain length (n) measured at different temperatures (T). The phase transition pressure (Pi(c)) depends linearly on the temperature. The models that account for the presence of the solvent lead to lower absolute enthalpy values for the two-dimensional phase transition at a given dPi(c)/dT value. It seems impossible to prefer definitely either of the models, mainly because direct experimental values for the yield of aggregation heat in Langmuir monolayers are currently not available. Taking into account the monolayer compressibility, the effect of the molar area of the amphiphile (a(0)) in the condensed phase is studied. The variation of the a(0) value for the surface pressures between monolayer collapse (0.53 nm(2)) and zero pressure (0.62 nm) affects only slightly the theoretical predictions of the behavior of the isotherms and the estimated values of the thermodynamic characteristics for the phase transition. However, the low a(0) value provides a slightly better description of the experimental data. The comparison of the effect of the side chain length (n) in the n-16PE monolayers reveals that for n = 2 the dPi(c)/dT values and the absolute values of the phase transition enthalpies are roughly two times higher than those characteristic for n = 1 and n = 14. Introduction and elongation of a side chain in the n-16PE causes up to medium side chain lengths an increasing disturbance of the lateral packing, but at long side chain length, the ordering is again improved as the conditions of a triple chain are approximated.