Pentenary Cu2ZnSn(SySe1−y)4 (kesterite) photovoltaic absorbers are synthesized by a one-step annealing process from copper-poor and zinc-rich precursor metallic stacks prepared by direct-current magnetron sputtering deposition. Depending on the chalcogen source—mixtures of sulfur and selenium powders, or selenium disulfide—as well as the annealing temperature and pressure, this simple methodology permits the tuning of the absorber composition from sulfur-rich to selenium-rich in one single annealing process. The impact of the thermal treatment variables on chalcogenide incorporation is investigated. The effect of the S/(S+Se) compositional ratio on the structural and morphological properties of the as-grown films, and the optoelectronic parameters of solar cells fabricated using these absorber films is studied. Using this single-step sulfo-selenization method, pentenary kesterite-based devices with conversion efficiencies up to 4.4 % are obtained.