The quasistatic and dynamic compressive strengths of transparent spinel (MgAl2O4) specimens with grain sizes of 0.4 and 1.4 μm are measured. The measured strengths and rate sensitivities of the spinel specimens are compared with another transparent ceramic material, AlON, with significantly coarser grains. The observed trends in rate sensitivities are consistent with micromechanical models that describe the dynamic failure of brittle solids under compression. Differences in observed crack speeds are also related to microstructural differences including grain size.