The reaction of LSitBu [L = PhC(NtBu)2] (1) with 4,4′-bis(dimethylamino)thiobenzophenone resulted in the [1+2]-cycloaddition product silathiacyclopropane 3, bearing a three-membered SiCS ring. The reaction of LSiC(SiMe3)3 (2) with 3,5-di-tert-butyl-o-benzoquinone leads to the [1+4]-cycloaddition product 4. In 3 the silicon atom is five- and in 4 it is four-coordinate. Compounds 3 and 4 were characterized by spectroscopic and spectrometric techniques. The molecular structures of 1, 3, and 4 were unequivocally established by single-crystal X-ray structure analysis.