Density-functional calculations have been performed to investigate the mechanism of dehydroxylation in both cis- and trans-vacant pyrophyllite. In agreement with predictions from experiment, calculations show that both form a trans-vacant dehydroxylated structure, with aluminum in trigonal-bipyramidal coordination and a highly distorted tetrahedral layer. Differences in the dehydroxylation behavior of cis- and trans-vacant pyrophyllite are due to the fact that in the former, adjacent hydroxyl groups bridge different pairs of aluminum atoms, while in the latter they bridge the same pair. These conclusions are thought to be valid for all aluminous dioctahedral 2:1 layer type clay minerals.