H₂S/OCS ratio in the 24 protostars of our sample. The class of each protostar has been represented using the color code in the legend. Sources are sorted by class, and then ordered by increasing bolometric temperature within each class. The discontinuous horizontal lines represent different upper limits to the H₂S/OCS ratio in ices from different regions: in black, W33A; in green, Mon R2 IRS2; in red, ChaMMSl. Only the OCS-rich sources are compatible with the more restrictive limit, ChaMMSl, which could give us some hints about the environment where the ice was formed.