Although previous work has provided qualitative evidence that the shapes of quartz sand grains can be modified significantly by postdepositional weathering and pedogenesis, the magnitude and rate of such changes have not been quantified. Fourier shape analysis of quartz grains from two generations of stabilized parabolic dunes from North Queensland, Australia, has shown that the grains in the older dunes are significantly more angular than those from the younger dunes. Because the sands of both dune generations have a similar provenance and transport history, the observed shape differences are attributed to postdepositional weathering in a humid tropical climate. This interpretation is supported by scanning electron microscope examination of grain surface textures and by textural analysis which revealed severe chemical etching and in situ silt production by disintegration of quartz grains. The observed changes in grain shape have been brought about in less than 8000 years.