A theoretical study of the SiC3+ isomers has been carried out, characterizing six different structures. Predictions for their geometries and vibrational frequencies have been made at both the MP2/6-31G* and B3LYP/cc-pVTZ levels. The nature of bonding in these isomers has been characterized by using Bader＇s topological analysis of the electronic charge density. According to our calculations, which range up to the CCSD(T)/cc-pVTZ level, the lowest-lying species is a rhombic structure, with the linear isomer and another rhomboidal isomer lying about 9.1 and 12.2 kcal/mol, respectively, higher in energy. Therefore, although SiC3+ is predicted to have a cyclic ground state,the linear isomer, and perhaps a second cyclic state, could be accessible to experimental detection. Both cyclic species are shown to be four-membered rings with only peripheral bonding, since there are no transannular Si-C or C-C bonds.