[en] The behaviour of two-phase materials (e.g. alloys) under irradiation is considered, and the following conclusions are reached: (a) equilibrium phase diagrams are not reliable indicators for either the composition or the existence of phases under irradiation; (b) phase stability is a function of the steady state defect concentration; (c) since the defect concentration affects the free energy of the solution, solute concentration variations will be coupled to defect concentration variations. This provides an explanation of segregation of solutes around defect sinks such as voids or surfaces. (U.K.)