Although ground-penetrating radar (GPR) is extensively used to characterize the regolith, few studies have addressed the effects of chemical and mineralogical compositions of soils and bedrock on its performance. This investigation evaluated the performance of GPR on two different granitic regoliths of somewhat different mineralogical composition in the San Jacinto Mountains of southern California. Radar records collected at an Alfisol Site were more depth-restricted than the radar record obtained at an Entisol Site. The Alfisol contains an argillic horizon whereas the Entisol has no horizon of clay accumulation. Also, the bedrock at the Alfisol Site, which contains more mafic minerals (5% hornblende and 20% biotite), is more attenuating to GPR than the bedrock at the Entisol Site, where mafic mineral content is less (<1% hornblende and 10% biotite). This variation in bedrock mineralogy, which does not change the rock classification, has a dramatic impact on the performance of GPR.