Mixed monolayers of octadecyltrimethoxisilane, OTMS, and poly(ethylene oxide), PEG, have been studied at the air/water interface as a function of pH by means of surface pressure and surface potential isotherms. PEO had a strong influence on the isotherms at large monolayer areas, while it was squeezed out of the air/water interface at high surface pressures. The LE-LC transition pressure of unhydrolyzed OTMS was increased by several mN/m in the presence of PEG, while PEO had little influence on the isotherms of hydrolyzed OTMS at small monolayer areas. The results are discussed in terms of attractive van der Waals interactions between methoxy groups of nonhydrolyzed OTMS and PEO.