For the low alloyed fine grained ferritic structural steel 15 MnNi 6 3 in a first step J-integral tests were performed at room temperature in argon, pure high pressure hydrogen gas as well as with admixtures of oxygen (10 vpm, 150 vpm) in the single specimen technique. The J(R)-curve received for pure hydrogen environment is essentially lower than the curve for argon. The J(R)-curve for hydrogen with 10 vpm oxygen is shifted towards the one for pure hydrogen whereas the admixture of 150 vpm oxygen results in a J(R)-curve near the argon curve. The J(Ic)-value for 10 vpm oxygen admixture is approximately twice the one for pure hydrogen, for 150 vpm oxygen admixture about three times that for pure hydrogen. For the specimen fractured in argon a dimpled fracture surface, for the one tested in pure hydrogen a flat quasi-cleavage fracture was observed. With increasing oxygen admixture the portion of ductile fracture grows.