Until recently, thiophene S-oxides have been elusive molecules. Their existence as intermediates in the peracid mediated oxidation of thiophenes to thio­phene S,S-dioxides, however, made it possible to use them in situ as dienes in Diels Alder reactions. Over the last 15 years, their concise synthesis and handling in pure form led to their utilization in further cycloaddition reactions. The high stereo-selectiv­ity of these reactions and the chemical versatility of the cycloadducts make thiophene S-oxides interesting study objects for the future.