The effects of cathodically charged hydrogen on the mechanical properties of pure aluminum have been studied through the use of tensile and microhardness tests. The effects have been further examined with TEM observations of the microstructural changes that accompanied the charging. Hydrogen charging was found to reduce the ductility and increase the yield and tensile stresses in tensile tests. Microhardness tests revealed that cathodic charging produced a severely hardened surface region. The hardness in this region quickly saturated, and further charging increased the depth of this region. TEM observations in this hardened layer revealed an increased dislocation density commensurate with the hydrogen charging. It was also found that the recovery of the hardened layer takes place in two stages: the first around 100 °C and the second around 350 °C.