High-pressure Raman and X-ray diffraction data on single crystals of hydrous layer-structure minerals, dickite and gibbsite, reveal structural phase transitions occurring between 2.0 and 2.5 GPa. In the case of dickite, this is the first documented occurrence of a pressure-induced phase transformation in a 1:1 phyllosilicate. Above the transition the structure of individual 1:1 layers remains unchanged, whereas the stacking of individual layers and the interlayer topology change significantly. Similar to dickite, single-crystal Raman spectra of gibbsite show large perturbations in the v(OH) region and suggest that the interlayer OH groups play a critical role in the phase transformation.