A simple model for first order phase transitions in monomolecular systems is developed and applied to (i) the condensed mesophase – liquid mesophase chain melting transition of phospholipid mono- and bilayers and (ii) a cooperative chain lifting phase transition found in monolayers of diesters with two polar heads. The excluded volume interactions are included in the model by requiring the polar heads to lie on a two-dimensional hexagonal lattice. The attractive van der Waals interactions are assumed to act between averaged molecular configurations of different surface area per polar head. Both the attractive intermolecular interaction and the description of the neighbourhood of the molecules are analyzed in the molecular field approximation. It is shown that the neighbourhood effects are negligible for the chain melting transition, which can then be described by a Ising model. They are, however, shown to be important for the chain lifting transition. The last section contains applications to experimental data and comments on the possibility of extending the model to the description of phospholipid mono- and bilayers with impurities.