We report the formation of heterogeneous OCS-water cluster anions [(OCS)(n)(H2O)(k)](-) (ngreater than or equal to1,n+kgreater than or equal to2), of which OCS-.H2O is the most interesting species in view of the near absence of unhydrated OCS- in the same ion source. The presence of OCS-.H2O indicates that the intra-cluster formation of OCS- does occur as part of the [(OCS)(n)(H2O)(k)](-) formation mechanism. In this light, the near absence of unhydrated OCS- anions points towards their metastable nature, while the abundance of the hydrated anions is attributed to the stabilizing effect of hydration. These conclusions are supported by the results of an extensive theoretical investigation of the adiabatic electron affinity (EA) of OCS. We conclude that the EA of OCS is either negative or essentially zero. The best estimate based on the Gaussian-3 theory calculation puts the EA at -0.059+/-0.061 eV. A study of the structure and energetics of OCS-.H2O predicts the existence of four structural isomers. Using the coupled-cluster theory, we find that the most stable structure is stabilized by 0.543 eV relative to the separated OCS-+H2O limit.