The association reaction S + CO OCS + hν has been identified as being particularly important for the prediction of gas-phase OCS abundances by chemical models of dark clouds. We performed detailed ab initio calculations for this process in addition to undertaking an extensive review of the neutral–neutral reactions involving this species which might be important in such environments. The rate constant for this association reaction was estimated to be several orders of magnitude smaller than the one present in current astrochemical data bases. The new rate for this reaction and the introduction of other processes, notably OH + CS OCS + H and C + OCS CO + CS, dramatically change the OCS gas-phase abundance predicted by chemical models for dark clouds. The disagreement with observations in TMC-1 (CP) and L134N (N) suggests that OCS may be formed on grain surfaces as is the case for methanol. The observation of solid OCS on interstellar ices supports this hypothesis.