We investigate the structure and decay of (OCS)(n)(-) cluster ions (n = 2-4) using photoelectron imaging spectroscopy. The results indicate the coexistence of isomers with OCS- and covalently bound (OCS)(2)(-) cluster cores. A several-fold decrease in the relative abundance of the dimer-based species is observed for n = 3 and 4 compared to n = 2. The OCS-(OCS)(n-1) cluster ions undergo direct photodetachment similar to OCS-.H2O, while (OCS)(2)(-)(OCS)(n-2) exhibits both direct electron detachment and cluster decomposition via ionic fragmentation and autodetachment. The autodetachment originates from either the excited states of the parent cluster or internally excited anionic fragments. It is described using a statistical model of thermionic emission, which assumes rapid thermalization of the excitation energy. A decrease in the relative autodetachment yield in the trimer and tetramer cluster ions, compared to the covalent dimer, is attributed to competition with ionic fragmentation. (C) 2003 American Institute of Physics.