[en] Full text: Maraging steels are ultra high strength alloys and have been widely utilized in engineering applications from pressure vessels operating in critical processes, aircraft components to military applications. The main alloying elements of these steels are Ni, Co, Mo and Ti. The characteristics of these materials are excellent formability in solution annealed or quenched condition, great ability to harden by aging, besides high fracture toughness. In this work the effects of hydrogen environment in an 18% Ni 300 grade maraging steel in the solution annealed and aged conditions were investigated. Samples were solution annealed at 820 deg C for 1h, air cooled followed by aging at 570 deg C for 3h. The microstructure was characterized by X-ray diffraction, eddy current, hardness measurement and optical and electron microscopy. To assess the effects of hydrogen on mechanical properties of the steel, slow strain rate tests (SSRT) were performed on annealed and aged samples. A strain rate of 1.0x10^-6 s^-1 was applied. Tests were carried out in air and with the samples immersed in electrolyte and exposed to a -1.2 V potential vs saturated electrode calomel. The results showed an elongation reduction from 10.96 to 6.59% and from 61.28 to 9.93% in area reduction for samples in the solution annealed condition. In the samples aged at 570 deg C for 3h, an elongation reduction from 9.68 to 2.55% was observed and from 30.80 to 5.51% in area reduction was seen, suggesting occurrence of hydrogen embrittlement. Furthermore, secondary cracks, perpendicular to the loading direction at the longitudinal surface of the solution treated samples were observed. Scanning electron examination showed a change in fractographic features from ductile dimples to quasi-cleavage and microvoid modes were also observed, confirming the fracture mode change from ductile to brittle with the presence of hydrogen. (author)