FTIR, Raman spectroscopy, and thermogravimetric analysis (TGA) were used to compare the structure and thermal properties of polyoctadecylsiloxane (POS) and octadecyl trichlorosilane (OTS) monolayers. The octadecyl chains in POS had higher conformational and intermolecular order than the same chains of OTS adsorbed as self-assembled monolayers (SAMs) on 106 nm hydrated silica beads. The latter were identical to the structures observed for OTS SAMs on superhydrated fumed silica that had primary particle sizes an order of magnitude smaller than the 106 nm beads, indicating that curvature did not affect the chain packing. The chains on POS were also more thermally stable to conformational and intermolecular disorder than the OTS chains, and this was attributed to increased free volume in the latter case. The differences in structure were partially determined by steric restrictions that arise because the Si-O-Si bond distance is less than the van der Weals radii of the alkyl chains. In POS, the most likely structure is therefore one in which the octadecyl chains are pointing in the same direction from every other Si atom, permitting good lateral chain packing. In the OTS SAMs, the octadecyl chains must all point in the same direction, away from the silica surface. Therefore, linear chains would be excluded but small clusters of dimers or trimers could be accommodated. These restraints increase the nearest neighbor distance between alkyl chains on SAMs and do not permit them to be as closely packed as for POS. This in turn contributes to the increased irreversible disordering of the chains with temperature.