Insoluble monolayer formed at the air/water interface compressed at a surface pressure above the equilibrium spreading pressure is unstable. In presented paper the classical theory for homogeneous nucleation is modified adding the activation energy term. It allows quantitative thermodynamic interpretation of the slow collapse phase transformation. The collapse of stearic acid Langmuir films has been carried out by systematic measurements of the area loss-time isobaric dependencies at various temperatures and isothermal dependencies at various pressures. Activation energies (activation enthalpies) and activation entropies have been evaluated for the nucleation (E-alpha = 1.32 eV) and the growth (E-beta = 1.55 eV) processes. The experimental data for various pressures are discussed on the basis of the Gibbs energy analysis. (c) 2006 Elsevier B.V. All rights reserved.