We have investigated the process of rehydroxylation of pyrophyllite as a limiting factor to the dehydroxylation upon thermal treatment. Car-Parrinello molecular dynamics simulations based on density functional theory have been used along with the metadynamics algorithm. Two possible rehydroxylation mechanisms reaction have been characterized, related to two possible intermediate structures along the rehydroxylation paths, and both involve the interaction of the apical oxygen atoms. At high temperature, the rehydroxylation reaction is highly competitive (free energy barrier (Delta F) = 1.5 kcal/mol) and inhibits the progress of the dehydroxylation reaction (Delta F = 40 kcal/mol). In addition to the rehydroxylation of the dehydroxylated structure, the water molecule supports the interconversion of the cross and on-site intermediates as well, Thus, rehydroxylation and interconversion among intermediates can justify the wide range of transformations as a function of the temperature observed experimentally.