Photo-induced microwave reflectometry (PIMR) has been used as a diagnostic aid in understanding the effect of ion implantation processing on defect concentrations in undoped LEC semi-insulating (S.I.) GaAs. Our measurements indicate over a 60% decrease in deep donor (EL-2) concentration after typical furnace anneal cycles, with less than a 30% decrease in net shallow acceptor concentration. These results are attributed to thermal dissociation of EL-2 and a reduction in gallium vacancy concentration under the furnace anneal conditions. With ion-implanted channels the PIMR signal increases appreciably with above-band gap illumination, indicating a reduction in surface recombination velocity. A long-response transient decay (~μS) is attributed to electron traps or multi-level centers introduced in the conducting channel and/or in the substrate region immediately beneath the channel. One-dimensional PIMR scans indicate a spatial periodicity of approximately 3 mm, attributed to thermal convection in the melt during boule growth. Implanted, annealed, and etched wafers indicate some change in defect concentration immediately below the conducting channel region which could result from implantation channeling. While additional research remains necessary to evaluate the generic applicability of these conclusions, PIMR is shown to provide additional insight into the effects of processing on S.I. GaAs wafers.