A method and theory presented earlier by the authors, for the straightforward measurement of resistivity in the surface region of charged insulators, in the initial phase of charge decay, is demonstrated also to be valid for much longer time scales. This is achieved with the use of a theoretical charge transport model, enabling a direct comparison between experimental and theoretical data. This comparison enabled the accurate determination of both the diffusion coefficient (D) and the layer of surface charge (&Dgr;z). Results are presented for glass, plexiglas (perspex), and polyethylene covering a useful resistivity range in the insulator class. The surface charge transport behavior of small rectangular samples is also studied using this computational model, focusing on the potential and charge distributions involved. From this study, some insight is gained into the way the charge in the surface region of insulators decays away with time when one end of a charged sample is earthed. The method provides a ready means of measuring the resistivity, carrier diffusion coefficient, and carrier occupancy depth in surface charged insulators. ©1996 American Institute of Physics.