Several reversed-phase materials for high-performance liquid chromatography were obtained by deposition of poly(methyloctylsiloxane) (PMOS) on HPLC silica particles, followed by immobilization using different procedures. Each phase had characteristic physicochemical and chromatographic properties. The present work evaluates the stability of these phases with both neutral and basic mobile phases. All of the stationary phases were quite stable to neutral mobile phase, with less stability at higher pH. However, one thermally immobilized phase presented high stability even at an elevated temperature with a pH 10.0 mobile phase.