An anomalous penetration of oxygen can be observed in certain neutron irradiated metallic samples. A strong oxygen diffusion occurs in niobium or iron but not in nickel, molybdenum or platinum. This diffusion can only be observed when the samples are water immersed during neutron irradiation. Different initial concentrations of oxygen in the samples do not yield significantly different results. The surface state or the crystalline state of samples are of negligible importance. The oxidation products, the nature of the radiations and the penetration of hydrogen were also studied.