A microphysical mechanism that may be important in the electrification of thunderstorms is probed in an experimental study of collisional charging of ice surfaces. The study examined mass and charge transfer in single collisions between two ice samples and their dependence on growth, temperature, impact strength, and time delay between impacts. The polarity is consistent with earlier results in that the more rapidly growing particle charges positively. We find that the magnitudes of charge and mass transfer increase with growth rate over an appreciable range, but the charge tends to saturate at high rates. The systematic dependence indicates that the charge transfer is more closely related to the difference in growth rate between the contacting surfaces rather than to their temperature difference. This result provides quantitative confirmation of earlier researchers' work. An appreciable time delay is required for the magnitude of the charge to recover from a previous collision. The magnitude of charge transfer cannot be explained by the thermoelectric effect or by any other mechanism proposed to date. It appears that the charge is carried in the mass that is transferred between the colliding surfaces, as predicted by Baker and Dash, <1989, 1994> but the magnitude of the mass transfer is much greater than can be due to ordinary surface melting, as proposed. ¿ 2000 American Geophysical Union