We provide a detailed in situ investigation of the two-dimensional polymerization of n-octadecyltri-methoxysilane Langmuir monolayers by PM-IRRAS (polarization-modulated infrared reflection absorption spectroscopy). The formation of the siloxane bonds at the headgroup level is unequivocally assessed by the appearance of a characteristic Si-O-Si band in the 1000-1150 cm(1) region. The aliphatic chains are shown to tilt toward the normal to the interface during polymerization. The process is sterically controlled and can be totally suppressed by replacing the hydrocarbon chain by its fluorocarbon homologous. The chain cross section is then too large to allow the reticulation of the silanol groups between neighboring molecules.