Free dianionic mixed silicon-carbon clusters SixCy2- (x=1,2 y=4-9) have been studied by standard ab initio methods. Branched, chainlike, and cyclic isomers have been investigated in detail. The smallest system that has been found to be clearly stable with respect to electron autodetachment and fragmentation is a triangular seven atomic SiC62- dianion, better referred to as Si(C-2)(3)(2-)(.) Chainlike isomers are less stable than the corresponding branched isomers. The onset of electronic stability within the chainlike structural class is predicted to occur at the eight atomic cluster dianion Si2C62-. Small stable cyclic isomers have not been found. The bonding mechanisms of the excess electrons in the different isomers are discussed and the systems are compared with other small gas-phase dianions. Furthermore, the essentially different principles of building up dianionic, monoanionic, and neutral mixed silicon-carbon clusters are explained and discussed.