The electrostatic surface potential of self-assembled monolayers (SAMs) of aliphatic and aromatic thiols has been measured using electrostatic force microscopy. The variation of the surface potential of chemisorbed alkanethiols, with respect to bare Au(111), is observed to increase with increasing chain length. The trend is similar to that observed in the literature. A preliminary theoretical model, based on treating the monolayer as a sheet of dipoles, has been used to calculate the surface potential of alkanethiols. Similar measurements on several aromatic thiols, with a symmetric and non-symmetric molecular structure, reveal that non-symmetric systems have significantly higher potential (≥ 170mV) than the symmetric molecules.