The synthesis of [4-C-14]-pelargonidin chloride and [4-C-14]-delphinidin chloride via [formyl-C-14]-2-(benzoyloxy)-4,6-dihydroxybenzaldehyde, omega,4-diacetoxyacetophenone and omega,3,4,5-tetraacetoxyacetophenone is described. The first step comprised labelling of the carbonyl group of 2-(benzoyloxy)-4,6-dihydroxybenzaldehyde, verifying that the coupling with omega,4-diacetoxyacetophenone or omega,3,4,5-tetraacetoxyacetophenone under hydrogen chloride atmosphere resulted in the formation of [4-C-14] labelled anthocyanidins. Copyright (c) 2006 John Wiley & Sons, Ltd.