Octadecyltrimethoxysilane (OTMS) was spread on the water surface of a Langmuir trough. Utilizing the high-intensity synchrotron X-ray radiation at the Advanced Photon Source at Argonne National Laboratory, the kinetics of film growth of OTMS as a function of the subphase pH was studied. In situ grazing incidence X-ray diffraction experiments at the air/water interface revealed that OTMS self-assembles into small domains and that the number of these ordered domains increases as OTMS undergoes hydrolysis/condensation reactions. The hexagonal packing of alkyl chains found in the OTMS monolayer is consistent with the ordering found in bulk OTMS by powder X-ray diffraction.