Hydrogen permeation of nickel-base heat-resistant alloys in a process gas environment is investigated in a high-temperature range up to 1273 K. Time-dependent permeation behavior of candidate alloys (R, NSC-1, SZ, KSN, 113M, and Hastelloy XR-51) for intermediate heat exchangers of a high-temperature gas-cooled reactor is examined in a reducing gas of 80% H2 + 15% CO + 5% CO2. The result in the reducing gas is compared to that of the permeation in pure hydrogen. For both measurements, a helium carrier gas method is used, simulating the practical configuration of the heat exchangers. The permeation rate decreased proportionally to the inverse of the square root of time in the reducing gas and had a square root dependence on hydrogen pressure at a constant thickness of the oxide layer. These results are discussed on the basis of a two-layer diffusion model.