The mechanism of proton diffusion in alkali metal hydrogen sulfates and selenates is analysed. The latest X-ray diffraction investigations of these compounds show that a highly correlated dynamic disorder of the hydrogen bonds occurs in the superionic phases. The observed diffraction patterns result from dynamically twinned crystals, which consist of a symmetry-related number of domains. We discuss the 'anomalous display of the dynamically disordered hydrogen atoms on the electron density maps', which is observed in the superionic phases. New X-ray diffraction data allow the atomic level mechanism of proton transport in the alkali metal hydrogen sulfates and selenates to be elucidated in detail.