This paper presents a definite correlation between hydrogen embrittlement and adsorption. The effects of the presence of gaseous additives on hydrogen embrittlement and hydrogen adsorption were studied. Those gaseous additives which halt a running crack in 4340 steel loaded in a hydrogen atmosphere also halt hydrogen adsorption. Those gaseous additives which accelerate the crack growth increase the supply of hydrogen atoms at the metal surface. It is concluded that the effect of gaseous additives to inhibit or promote crack growth is a consequence of their ability to increase or decrease the supply of hydrogen atoms at the metal surface by some chemical process.