Summary: Sunlamp irradiation of phenacyl sulfides PhCOCH2SCHRR'affords thiocarbonyl compounds S= CRR'that can be trapped in high yield by using the nitronate CH, CH= N+(OTBS) O-; the heterocycle 3 resulting from 1, 3-dipolar cycloaddition is cleaved rapidly by fluoride ion to give ketones or aldehydes.