Self-assembled monolayers (SAMs) consisting of twenty-two carbon, methyl-terminated alkanethiolates adsorbed on vapor-deposited gold have been fluorinated in vacuum using an effusive F atom source. The reactive uptake of fluorine as a function of F atom exposure was calibrated using X-ray photoelectron spectroscopy. The surface potentials (V-s) of SAMs that were fluorinated to different degrees were measured as a function of temperature using a high-sensitivity vibrating probe electrostatic voltmeter, The surface potential grew increasingly negative with increasing fluorine uptake, reflecting the charge asymmetry that is induced in the alkanethiolate chains as a result of the substitution of fluorine for hydrogen. The V-s of the most highly fluorinated SAMs displayed a negative temperature dependence. This observation may be indicative of a pyroelectric effect in these monolayers although a definitive conclusion awaits further measurements.