The chalcopyrite compound CuGaSe2 is considered as potential absorber material for thin film solar cells due to its suitable optoelectronic properties. We investigated intentionally undoped bulk material that is used as source material in a thin film deposition process by electron-paramagnetic resonance (EPR) and light-induced EPR (LEPR). Both methods are well suited to determine the concentration and electronic activity of transition metal ions in the chalcopyrite host lattice. The recently identified N+ centre was found by LEPR to act as recombination centre for photogenerated charge carriers. Moreover, the typical signature of Cu2+ could be detected in EPR spectra of aged CuGaSe2 powders indicating an oxidation process to which the material is subjected at ambient conditions. The ageing process is significantly accelerated in water-saturated air of 100% humidity compared to dry air of 50% humidity. The connection of this microscopic process with the electronic degradation of the material during solar cell processing will be discussed.