We report on a continuous flow synthetic process of non-symmetrical ureas, based on the implementation of a Staudinger/aza-Wittig reaction sequence. This flow technology allows obtaining the key urea functional group directly from the fundamental C1 building block CO 2 . A library of 26 derivatives, including alkyl/alkyl, alkyl/aryl and aryl/aryl ureas could be synthesised. In addition, the method was also suitable for the synthesis of S-thiocarbamates. The potential for large-scale reaction implementation and a first application to continuous flow 13 C-isotope labeling are shown.