The effect of humidity on the adhesive properties of a model pressure sensitive adhesive (PSA) composed of poly(ethylene propylene) (PEP) and the n-butyl ester of abietic acid was studied using lateral modulation (X-modulation) measurements with a scanning probe microscope. The lateral force on a probe oscillating laterally in the "stick" regime decreases slightly with increasing humidity on the hydrophobic surface of the pure polymer matrix. On the homogeneous, hydrophilic surface of the adhesive loaded with 60 wt % tackifier (PEP/60), the corresponding lateral force grows strongly with increasing humidity, To better understand how the humid environment affects the nanomechanical properties of the adhesive surface, the X-modulation technique was combined with a force-distance curve measurement to yield a more precise and reproducible characterization, The properties of the surface of an aged PEP/60 sample were much more sensitive to variations in humidity than were the properties of the homopolymer matrix by itself. A large oscillating lateral force in X-modulation and small stiffness in a force-distance curve were observed for PEP/60 at high humidity. These observations are consistent both with a model that envisions a tackifier-enriched region within 50-100 nm of the surface and with the hypothesis that at a hydrophilic surface water molecules may strongly alter the surface-tip interactions or modify the mechanical properties of the material nearest the surface.