The recent demonstration of “charging microscopy” to reveal resistivity variations in semi-insulating GaAs has generated interest in using the scanning electron microscope (SEM) as a contactless characterization technique for GaAs wafers. Resistivity variations give rise to a surface potential and secondary electron (SE) contrast, or “charging microscopy”, when viewing a wafer in the SEM. In this work we have shown that operating conditions may play a significant role in the SE image. Varying accelerating voltage from 5 to 35 keV gave rise to minor changes in the SE image, but varying beam current from 2×10−11A to 2×10−6A resulted in dramatic changes in the SE image, with two image contrast reversals occurring. The explanation for these changes in the SE image are unclear, but factors that are involved include surface and bulk conductivity, beam induced dielctric relaxation, and the sample-detector geometry.