Nanoparticulate zirconium tungstate prepared through hydrothermal methods was found to autohydrate under ambient conditions. This results in positive thermal expansion, limiting its usefulness for controlled thermal expansion composites. TEM and BET studies provided strong evidence that the cause of autohydration is a result of structural defects present in the nanoparticles, while kinetics are governed by surface area, suggesting that processing methods can be used to minimize or overcome this problem.