Abstract A phenomenological traction-separation law that describes the cohesion of an inclusion/matrix interface in the presence of hydrogen is suggested such that the associated reversible work of separation during fast decohesion is exactly equal to that predicted by the thermodynamic theory of Hirth and Rice (Metall. Trans. 11A (1980) 1501) and Rice and Wang (Mater. Sci. Eng. A 107 (1989) 23) in the corresponding limit. The law is used to study interfacial debonding around an elastic inclusion imbedded in an elastoplastically deforming matrix while transient hydrogen transport takes place in the matrix, the inclusion, and the opening interfacial channel. Interfacial separation is modeled through cohesive elements and is simulated incrementally within the updated Lagrangian formulation scheme used to model bulk material elastoplasticity. For material data pertaining to nickel-base alloy 690, the numerical results indicate that both hydrogen-induced reduction of interfacial cohesion and matrix-softening lead to a reduction of stress at which void nucleation commences relatively to case of a hydrogen-free material. On the other hand, there is a competitive effect on the void nucleation strain: while cohesion reduction decreases this strain, matrix softening increases it, and its final value depends on the outcome of this competition. Thus the suggested model of the hydrogen effect on cohesion, although calibrated in accordance with the fast-separation limit (small cohesion reduction) of the Hirth–Rice–Wang theory, does allow for internal material failure with a clear and substantial effect on the external macroscopic loads.