This paper explores the effects of hydrogen on the mechanical properties of a 2 1/4Cr-1Mo steel. The results for both microstructural conditions, as received and aged, indicated a loss of ductility after hydrogen charging treatment, but the yield strength and ultimate tensile strength remained unaltered. The fractograph analysis revealed that the fracture mode was modified by the hydrogen. The steel in the as-received condition showed craters and fisheyes on the fracture surface. The aged steel showed a brittle appearance associated with cleavage facets and small portion of areas with dimples. The hydrogen diffusivity and solubility were investigated using electrochemical permeation technique. It was observed that the hydrogen diffusivity decreased from 2.3 +/- 0.4 x 10(-10) m(2) s(-1) in the as-received condition to 5.7 +/- 0.1 x 10(-11) m(2) s(-1) in the aged condition. The hydrogen solubility showed an increase for the aged condition in comparison to the as received sample. Both phenomena can be attributed to carbide evolution during aging, resulting in an increase of the carbide/matrix interfacial area.