Abstract Highly resolved spectra of transition metal ions in II–VI compounds show extremely narrow zero-phonon lines to that isotope effects can be observed. An adequate description of the observed fine structures requires the inclusion of an intermediate Jahn-Teller effect using local vibrational T 2 modes. The observation of different isotope splittings at Ni impurities in CdS leads to the assumption that different local vibrational modes are coupled to the electronic states of that system and that the local vibrational modes have different dependencies on the mass of the impurity. The calculated energy shifts are in agreement with the observed isotope shifts of natural and enriched Ni and Cu doped II–VI compounds. The influence of uniaxial stress, magnetic field and temperature on the isotope shifts is also investigated.