This paper presents research of the hydrogen gas effects on physical and mechanical properties of the precipitation-hardened nickel alloys EP741 and EP99 at pressures up to 65 MPa and temperatures up to 840 degrees C. It has been shown that conventional yield strength, tubular-shaped sample fracture stress and long-term strength are not practically affected by hydrogen. The characteristics of alloy ductility are more susceptible to hydrogen effect. EP741 alloy Is especially susceptible to the hydrogen environment effect at temperature of 460 degrees C, and EP99-at temperatures of 20 degrees C and 460 degrees C. Hydrogen permeability relationships for alloys in a wide range of temperature and pressure were obtained.