Low, stabilized sand dunes cover much of the Cactus and La Posa Plains in western Arizona, near the town of Parker. The provenance of the dunes is not well defined, and it had been hypothesized that they were possibly related to extensive eolian sand deposits in the eastern Mojave Desert of California, on the other side of the Colorado River. Major oxide analyses of bulk sand samples collected from southeastern California and western Arizona show clear chemical differences. The Mojave sands all have SiO2 < 79 wt%, Al2O3 > 10 wt%, and Na2O + K2O > 5 wt%, whereas the Colorado River and Arizona sands have SiO2 > 81 wt%, Al2O3 < 6 wt%, and Na2O + K2O < 4 wt%. These results show that the stabilized dunes near Parker are chemically indistinguishable from Colorado River sands. The chemical differences between the southeastern California sands and the western Arizona sands are supported by X-ray diffraction results that indicate these two sand populations have significant differences in the relative abundance of quartz and feldspar. Our data also indicate a near-source origin for the Mojave sands, whereas the Colorado River and Arizona sands have undergone more prolonged chemical weathering. The major oxide results are consistent with published trace element studies by others that clearly distinguished between mature and immature sands in California and Colorado. The quartz/feldspar ratio may be amenable to measurement by some advanced remote-sensing instruments.