The local contact potential difference (CPD) between different linear alkanethiols self-assembled as monolayers on Au substrates was investigated with Kelvin probe force microscopy (KPFM). Our results demonstrate that KPFM simultaneously provides information on the sample topography and its contact potential down to a lateral resolution of 100 nm. The reported CPD measurements allow the distinction between regions in the monolayer comprising thiol molecules with chemically different terminal head groups down to a resolution of 3 meV. Furthermore, the CPD values measured in monolayers having one type of functionality vary with the chain length of the molecules in the film, which is consistent with a dipole-layer model. Variations in the length of the alkyl chain reveal a linear dependence as a function of the (-CH₂-) units in the chain, with an increase in the CPD potential of 14.1 ± 3.1 mV per (-CH₂-) unit.