The interaction of hydrogen isotopes with MANET (MArtensitic for NET) has been studied by a gas-evolution method in the framework of activities aimed at characterizing this steel. Temperatures in the range 573 – 873 K and loading pressures between 103 and 105 Pa have been used. In the temperature and loading pressure range studied, hydrogen and deuterium diffusivity in MANET is about two orders of magnitude higher than for AISI 316L (austenitic steel), ie in the range from 10−9 to 10−8 m2 · s−1. However, the solubility (Ks) in MANET is about an order of magnitude lower than in the austenitic steel, ie in the range 10−3 to 10−2 mol· m−3 · Pa−1/2. Changes of these properties caused by a phase change of the material at temperatures above 673 K are discussed. The hydrogen and deuterium data obtained were used to calculate the tritium solubility and diffusivity data by means of quantum-statistical theories.