We report contact potential difference (CPD) measurements of n-alkanethiol self-assembling monolayers (SAMs) adsorbed to Au substrates using an organomercaptan-modified Au probe. We perform experiments by applying a triangular-sweep voltage between the sample and probe while measuring the resulting electrostatic force. The interfacial force microscope permits us to keep the probe/sample distance rigidly fixed, which allows us to directly measure the electrostatic force at a constant interfacial separation. The CPD is determined by measuring the applied potential necessary to null the electric field and eliminate the interfacial force between the two surfaces. We show that CPD values obtained using Au probes modified with methyl-terminated SAMs are stable and reproducible, whereas identical unmodified probes yield highly variable data. Our experimentally determined CPD values are in qualitative agreement with calculated CPDs for several different omega-terminated SAMs.