[en] A theoretical study of the isotopic-mass dependence of the internal transitions of Fe2+ at a cation site in a cubic zinc-blende semiconductor is presented. The model used is based on crystal-field theory and includes the spin-orbit interaction and a weak dynamic Jahn-Teller coupling between the (5) Gamma(5) excited manifold of Fe2+ and a local vibrational mode (LVM) of Gamma(5) symmetry. The mass dependence of the LVM frequency is described, in the harmonic approximation, within two different limits: the rigid-cage model and a molecular model. In the rigid-cage model, the Fe2+ ion undergoes a displacement but the rest of the lattice is fixed. In this case, a simple M-1/2 dependence of the frequency is obtained and the Jahn-Teller energy, E-JT, is independent of the mass. In the molecular model, the four nearest neighbors of the magnetic ion are allowed to move and the LVM then behaves as the Gamma(5) mode of a MX4 tetrahedral molecule leading to a more complicated dependence of the frequency on the isotopic mass and to a mass-dependence of E-JT. The theoretical results obtained with these two models are compared with the observed isotopic shifts of the zero-phonon lines in InP:Fe and GaP:Fe corresponding to an optical transition between the vibronic Gamma(1) ground state and the lowest Gamma(5) state originating from the (5) Gamma(5) excited orbital multiplet. A prediction of the isotopic shifts of the zero-phonon line in GaAs:Fe is also presented. (C) 1997 Elsevier Science Ltd.