Hydrostatic pressure effects on cathodic deposits and long-term hydrogen permeation in 0.2 mol/L NaOH, 3.5% NaCl and artificial seawater were characterized by the high-pressure Devanathan cell hydrogen permeation tests, electrochemical impedance spectroscopy, Raman spectroscopy, scanning electron microscopy, coupled focused ion beam lithography and energy dispersive X-ray spectroscopy. Hydrostatic pressure accelerates the hydrogen evolution reaction and causes hydrogen atoms to permeate into steel. Furthermore, hydrostatic pressure increases the interfacial pH; changes deposits from a mixture of brucite, aragonite, and calcite to brucite only; and inhibits hydrogen permeation into steel in artificial seawater. These competitive effects control the mechanisms of hydrogen entry into steel in deep seawater. Published by Elsevier Ltd.