A new method has been developed to measure the Kelvin contact potential difference (CPD) of insulating submicron particles as a function of relative humidity (RH). The insulative particles are dry blended onto the surface of conductive particles at a high relative humidity and the resultant mixture is conductive, thus, enabling a CPD measurement that is reflective of the insulative coating. A second modification to the standard CPD methodology is the use of water as an RH-independent reference. Using these two new modifications to the Kelvin measurement, the CPD of common xerographic toner additives such as silica, titania, and alumina particles have been determined as a function of RH. The CPD values of these insulative metal oxide particles are compared to the triboelectric values obtained for the same particles used as surface additives on a xerographic toner. A good linear correlation exists between the charge of the additive coated toner and the CPD measured as a function of RH. Both the charge level and the RH sensitivity of the charge with the metal oxides can be explained by changes in the metal oxide contact potential. Thus, the mechanism of charging in these metal oxides is dominated by the transfer of electrons between states that can be described in terms of work functions. Also concluded is that ion transport has a minor contribution in the mechanism of triboelectric charging with these metal oxides.