MP4(SDTQ)/6-311++G(d,p) and B3LYP/6-311++G(d,p) calculations were carried out to optimize the structures of S-ylide compounds. The C-S bond characters were discussed by topological analysis of the electronic density. The following conclusions are drawn: (1) Neither the S-ylide nor the S-ylide-like radical has C_s symmetry. (2) The C-S bond character of S-ylide-like radical is similar to that of S-ylide, there exist π bonds in both types of ylide compounds. However, there are two electrons in the π bond of S-ylide, while there is only one electron in the π bond of S-ylide-like radical. (3) The electron in the π bond of S-ylide-like radical appears mainly near the C atom. It has π(C → S) bond character, therefore it is weaker than the C-S bond in the corresponding product.

Key words: S-ylide, TOPOLOGY, CHEMICAL BONDS, ELECTRON DENSITY, CONFIGURATION