Several reactions of a-thioxo ketones and a-thioxo thioamides with diazo compounds were investigated. Most of them proceeded via a reactive thiocarbonyl ylide, which underwent a [2 + 3] cycloaddition with the a-thioxo ketone to give the Schçnberg products 17 -19 or a 1,5-dipolar electrocyclization to give the corresponding five membered 1, 3-oxathioles (i.e., 13, 20a, 20b, 21, and25) and1 ,3-dithioles (i.e., 33, 34, and 35), respectively. In the case of thioamide 32, the thiocarbonyl ylides underwent a competitive 1,3-dipolar electrocyclization to yield the corresponding thiiranes. In these cases, spontaneous desulfurization led to the corresponding alkenes 36 and 37. The nature of the employed thiocarbonyl or diazo compounds seem to be decisive for the reaction pathway. When the diazo compound bears at least one H-atom in the b-position to the diazo group (i.e., diazocyclohexane 15f), no products of an electrocyclization could be isolated in the reactions with a-thioxo ketones or a-thioxo thioamides. The only products which were isolated in these cases were 2-[(cyclohex-1-enyl)sulfanyl]-1,2-diphenylethanone ( 22) and 2-[(cyclohex-1-enyl)sulfanyl]-N,N-dimethyl-2-phenylthioacetamide (38), which were formed by a [1,4]-H shift of the corresponding thiocarbonyl ylides.