Stable crystalline phosphorus ylides were obtained in excellent yields from the 1:11 addition reaction between triphenylphosphine, dialkyl acetylenedicarboxylates, and strong NH-acids, such as 7-azaindole. These stabilised phosphoranes exist as a mixture of two geometrical isomers as a result of restricted rotation around the carbon-carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group.