Microwave photoconductive decay is widely used to measure the recombination lifetime in semiconductors and is generally considered to be a low-injection technique. Here, the authors examine the technique in detail from both a theoretical and experimental perspective. When the skin or penetration depth is much less than the sample thickness, the reflectance-conductivity relationship is nonlinear when the injected carrier density exceeds about 5% of background doping. However, when the penetration depth is much larger than the sample thickness, the transmitted signal is linear over several orders of magnitude of sample conductivity. In the transmission mode of operation, high-injection lifetime measurements are accurate. (C) 2008 American Vacuum Society.