The interfacial behavior of products used as food emulsifiers depends on the aqueous-phase composition. In this work, we studied the subphase pH influence on structure, stability, and rheological parameters for monostearin monolayers spread on the air-water interface. An expansion was observed in the film structure for the acid aqueous phase, which was higher when pH was reduced. Monolayers are even more expanded when the aqueous phase is alkaline, but the expansion decreased as the pH increased. These effects may be due to changes in water dipole distribution around monostearin polar groups and, therefore, in the hydrogen-bond number between water molecules and between these and monostearin headgroups. The films were essentially elastic. The surface rheological properties depended on the monostearin concentration at the interface, but were not very frequency-dependent, The stability or the rheology of the films did not change significantly because of variations in the pH subphase.