Hydrolysis and condensation reactions of octadecyltrimethoxysilane (C(18)TMS) and octadecyldimethylmethoxysilane (C(18)DMMS) have been studied by means of the Langmuir-Blodgett technique and electron spectroscopy for chemical analysis (ESCA). C(18)TMS can form linear two-dimensional polymers, while C(18)DMMS forms only dimers. The polymerization/dimerization rate seems to increase in the sequence pH 5.6 --> 11 --> 1. At pH 11 the spread monolayer contracts when electrolyte is added and becomes less compressible. This indicates decreased electrostatic repulsion within the ionized monolayer and/or a higher level of polymerization/dimerization of the film molecules. C(18)DMMS could be compressed at pH 1 to a surface area of similar to 18 Angstrom(2), before the film enters a condensed phase. Constant surface pressure-area relaxation experiments indicate that a nucleation process takes place in the C(18)DMMS film at pH 1 for pi=20 mN m(-1). In contrast, the C(18)TMS film at pH 1 was stable for more than 12 h at 40 mN m(-1). When two subsequent compression steps are applied with a delay of 16 h, the C(18)TMS films (pH 5.6 and 11) reproduced the condensed pH 1 film, with molecular areas of similar to 20 Angstrom(2), while the C(18)DMMS film (pH 5.6 and 11) was still in an expanded state. A condensed state in the two-dimensional film indicates good packing and strong cohesion forces between the hydrocarbon chains. The dimer-forming C(18)DMMS will not achieve the same strong cohesion that we find in the fully polymerized C(18)TMS films. For the C(18)TMS film at pH 11 a lowering of the barrier speed increased the polymerization rate, as observed by decreasing compressibility and decreasing areas per molecule for a given surface pressure. Langmuir-Blodgett films analyzed by ESCA confirmed these results.