The stress–strain equation of state for a material with dissolved hydrogen is derived within a twocomponent continuum model. It is demonstrated that, for the parameters corresponding to conventional metals, it has the shape of a loop similar to the van der Waals loop. On this basis, it is concluded that hydrogen embrittlement of materials under load is a typical first-order phase transition and, therefore, occurs through nucleation of a new phase. Thermodynamic and kinetic theories of this transition are developed, and time dependences of all the main characteristics of this process are calculated.