Mixed self-assembled monolayers have been used as a model surface to study the adsorption of DMMP (dimethylmethylphosphonate) and water vapor under conditions relevant for sensor development. The use of -OH and -CH3 terminated thiols allows us to vary the polar character of the surface in a wide range. Using a surface acoustic wave sensor it is found that the adsorption equilibrium for both vapors are strongly affected by the surface hydrophilicity. Further analysis of the measured water vapor isotherms allows us to extract information about the strength of adsorption. It. is found that at a surface composition of approximately 1:1 a transition occurs. DMMP vapor at concentrations in the ppb range is found to produce adsorption isotherms with saturation corresponding to less than one monolayer. In this case in-situ IR measurements of the adsorbed molecules reveals that the strength of the hydrogen bonds involved is independent of surface -OH density.