Using radioactive isotopes of shallow dopants (Ag, As, Rb) as well as of native or isoelectronic elements (Se, Te, Cd) which were incorporated as host atoms and then transmute into relevant dopants (transmutation doping), we investigated the doping phenomena occurring in wide band-gap II-VI compounds CdTe, ZnTe and ZnSe by the classical methods of semiconductor physics Hall effect, C-V and photoluminescence measurements. Thus, we could assign unambiguously defect features in electrical and photoluminescence measurements to extrinsic dopants by means of the half lives of radioactive decay. In As-doped ZnSe samples we observed two states: a metastable effective mass-like state and a deep state. The occurrence of the latter state is always linked with the high resistivity of As-doped ZnSe crystals. The transmutation doping experiments reveal, that the so-called self-compensation being typical for wide band-gap II-VI compounds can be overcome if the thermal treatment for dopant incorporation is time separated from its electrical activation, here due to transmutation at room temperature.