CuInS2 absorber layers for thin-film solar cells are examined in this work. The influence of post-deposition annealing in hydrogen and oxygen atmosphere is studied by means of photoluminescence (PL) and nuclear reaction analysis (NRA). The intensity of a PL peak at 1.445 eV can be drastically influenced by post-deposition treatments. This transition is ascribed to the donor-acceptor pair recombination between a sulfur vacancy and a copper vacancy. From the measurements, a simple defect model is deduced which assumes the occupation of sulfur vacancies by oxygen. The sulfur vacancy can be activated by hydrogen annealing and passivated by oxygen annealing.