The measurement of lateral force in response to small amplitude lateral oscillations of a sample using a scanning probe microscope (X-modulation) is presented as an effective means of identifying key qualitative differences in the nanomechanical behavior of adhesive surfaces. The technique itself has recently been developed by others, but is applied here to the study of pressure-sensitive adhesives (PSAs) for the first time. A model PSA of poly(ethylene propylene) and n-butyl ester of abietic acid was studied. The characteristic features of "stickiness" and elasticity manifest themselves in the presence or absence of a "stick-slip" transition and changes in the forcing amplitude at which this transition takes place, While conventional lateral force microscopy imaging of a heterogeneous PSA surface leaves the impression that the polymer-rich matrix regions are responsible for the adhesiveness of the surface, X-modulation measurements make it clear that the tackifier-enriched domains provide the adhesiveness. X-modulation measurements also indicate that the homogeneous surface of the adhesive loaded with 60 wt % tackifier is more "sticky" than the more adhesive domains in a heterogeneous surface of a PSA with higher tackifier loading. The tackifier loading (60 wt %) giving the best overall performance as indicated by microindentation, which is sensitive to bulk rheology, also corresponds to the best adhesiveness right at the surface of the adhesive for this system.