Stimulated by the unusual phase diagram of the superconducting states in U]_,.Th,.Bei3 which furnished a renewed debate about the nature of the heavy-fermion superconduct¬ ing order parameter (conventionally anisotropic vs. unconventional), we have initiated a study of the effect of impurities on the superconducting state of CeCu2Si2. We conclude a 'diamagnetic' pair-breaking effect by the non-magnetic impurities La and Y and an additional "paramagnetic" one by Gd. Comparison with low-temperature resistivity results in the normal state leads to the assumption that the Tc — depression for vanishing dopant concentration is determined by a highly anisotropic scattering of the heavy quasiparticles off these impurities
