Photochemical interpretations of slow (opto-electronic) phenomena in CdS crystals are investigated theoretically and experimentally. It is seen that the presence of Ni impurity atoms has a major influence on some of these slow phenomena. Consequently, impurity atoms must be included in the considerations if a model for photochemical bulk reactions is to be developed. A non-photochemical interpretation is possible for the phenomenon of the slow decrease of the photocurrent at constant illumination, if this decrease is connected with thermal quenching. For the slow changes in the spectra of thermally stimulated conductivity, which form the main body of the experimental part of this study, a photochemical interpretation seems to be more adequate. A working hypothesis is drawn up: with the aid of isovalent impurity atoms photochemical reactions in CdS can already occur a little below room temperature. This was tested by experiments on crystals doped with Cu or Ag and to which in addition (isovalent) Ni was added. The recording of the thermally stimulated curves on a logarithmic scale over eight decades showed many new details. So it was found that an intimate correlation exists between all parts of such a curve. This is explained by means of a model of donor-acceptor pairs, statistically distributed among various lattice distances. By ascribing traps to these pairs a theory is possible which gives a quantitative description of the low-temperature region and a qualitative one ofthe high-temperature region. On the base of the pair model the stationary situation after long illumination is described. The pair model gives a better explanation of the glow curve as a whole than the model of slow recharging of a double acceptor does.