We report a new electrometer which allows the study of contact charging between a single microparticle and a metal surface. The electrometer is based on an electrodynamic levitator-trap and has a leakage current of about 10-23 A. The electrometer is applied to the study of the contact charging of spherical polystyrene-divinyl benzene copolymer particles against a Ni surface under ambient and a "dry" N2 atmosphere. Significantly reduced charge exchange after eliminating water molecules from the particle would be strong evidence for an ionic charge exchange mechanism. Through a combination of gravimetric and light scattering measurements we have determined both the amount of water accreted in a humid atmosphere and the maximum amount of water that could be on the surface of the particle. Our experiments indicate approximately constant charging, independent of the amount of water on the particle. © 1990.