The dependence of the size of the cooperative unit (C.U.) of amphiphilic molecules on surface pressure (pi) in the liquid expanded (LE)-liquid condensed (LC) phase coexistence region of Langmuir monolayers has been formulated and calculated using measured isotherm data. The C.U. size changes largely depending on the surface pressure in the coexistence region: these submicroscopic molecular aggregates are not static objects, but dynamic ones characterized by large fluctuations in size. It has been found that the C.U. size distribution can be a natural consequence of the significant change of monolayer compressibility, which reflects large molecular area density fluctuations, in the coexistence region. (C) 2012 Elsevier Inc. All rights reserved.