Laboratory measurements of the sign of electric charge transfer separated in rebounding collisions between large ice crystals and a graupel pellet growing by riming are analyzed. In these experiments, the usual negative rimer charging regime noted at temperatures below about -10¿C at moderate liquid water contents is reversed to positive when the crystals have grown larger. The experiments were performed with an impact velocity of 5.5 m s-1, the ambient temperature was varied in the range -15 to -26¿C with an effective water content from 0.5 to 3 g m-3. A cloud particle imager shows a clear correlation between the rimer charge sign reversal to positive and the presence of a high proportion of irregular and large ice crystals or aggregates in the cloud. The results suggest that in collisions between graupel and these ice particles the graupel will charge positively while the irregular particles carry away a negative charge. These are the first measurements of this kind of collision and the findings may have important implications in the thunderstorm electrification process.