The effect of plasma nitriding on hydrogen permeation through AISI 4340 was studied both experimentally and theoretically. Effective hydrogen diffusion coefficients were measured for three different AISI 4340 membrane specimens: as-received, nitrided, and nitrided specimens with the compound layer removed by mechanical abrasion. The as-received specimen had a higher diffusion coefficient (2.8 x 10(-6) cm(2)/sec) than the nitrided specimens. The nitrided specimen with the compound layer intact had a much lower hydrogen diffusion coefficient (0.65 x 10(-6) cm(2)/sec) than the specimen without the compound layer (1.9 x 10(-6) cm(2)/sec). These results demonstrate that the gamma'-Fe4N rich compound surface layer and the N that diffuses more deeply into AISI 4340 during plasma nitriding both. reduce the effective hydrogen diffusion coefficient. Multiple permeation transients yield evidence for the presence of only reversible trap sites in as-received specimens and the presence of both reversible and irreversible trap sites in nitrided specimens, with and without the compound zone intact. In addition, density functional theory-based molecular dynamics simulations yield hydrogen diffusion coefficients through gamma'-Fe4N (1.5 x 10(-5) cm(2)/sec) one order of magnitude lower than through alpha-Fe (1.2 x 10(-4) cm(2)/sec), which supports the experimental measurements of hydrogen permeation. (C) 2014 The Electrochemical Society. All rights reserved.