Ratio of H₂S/OCS 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. Panel a) shows the ratios calculated with the estimated column densities of H₂S and OCS. Panel b) shows the same data but assuming a worst-case scenario where all protostars with OC³⁴S detection but no ${\text{H}}_2^{33}\text{S}$ detection have their H₂S column density scaled by a factor 37, to take into account possible optically thick emission in the H₂S line. The scaled ratios have been represented with a downward triangle, which is a superior limit to the maximum expected H₂S/OCS ratio after accounting for the opacity. In this case, the two differentiated trends still appear.