Reactive magnetron sputtering from copper and indium targets in argon-hydrogen sulfide mixtures has been used to prepare CuInS2 absorbers for thin film solar cells. Compared with the sequential process, where sputtered metal films are sulfurized, this technique offers the advantage of a one-step process. The influence of substrate temperature, total sputtering pressure and the copper-to-indium ratio on structure and morphology was investigated. The argon-to-hydrogen sulfide ratio was fixed at 1: 1. Lower sputtering pressure and higher substrate temperature lead to larger crystal grains. The films were prepared copper-rich and exhibited only the CuInS2 (roquesite) and the CuS (covellite) phases. Due to the phase separation, known from the growth of copper-rich CuInS2 films, the metallic CuS phase on top of the as grown films could be removed by wet-chemical etching. As in the case of the sequential process or of coevaporated films, the copper-to-indium ratio has a strong influence on grain size and electronic properties. First solar cells prepared with these absorber films exhibited a maximum efficiency of 6.4% at AM1.5 illumination. The diffusion length in the first cells prepared from reactive magnetron sputtered absorbers are three to four times lower than that of coevaporated CuInS2 films. However, this ion assisted deposition process offers the possibility to prepare absorber films at lower temperatures compared with a thermally activated process, like sulfurization or coevaporation. Furthermore, the presented results are a further step towards a continuous vacuum process for CuInS-thin film solar cell preparation.