Force-area, surface potential-area and viscosity-surface pressure data were obtained for stearic acid monolayers on water and heavy water (DOD) at 15, 25 and 37°C. These data show that substitution of DOD for HOH increases the surface potential of stearic acid monolayers and increases surface viscosity. However, no differences in force-area curves were detected between monolayers on HOH and DOD.