The influence of grain size distribution on the achievement of the chemical equilibrium between the solid solution region and the hydrogen gas phase for the St909 alloy isotherms has been investigated for two different grain size distributions to test the possibility of using this alloy without a previous cycling treatment. The small grain size distribution gave ΔH=−33.54±1.03 kJ/mol H , ΔS=−18.31±3.43 J/K mol H in a H 2 concentration range 0.05≤x≤0.09, and ΔH=−42.30±1.06 kJ/mol D , ΔS=−21.78±4.49 J/K mol D in a D 2 concentration range 0.02≤x≤0.09. A comparison with ΔH and ΔS values in the infinite dilution range for H 2 and T 2 obtained for the conditioned St737 alloy suggested that the small grain distribution gave H 2 pressure values close to but not yet at equilibrium. A qualitative inquiry on the possible factors limiting the achievement of equilibrium is also presented and suggests surface dissociation phenomena as the most probable cause.