1,2-Alkylidenediphosphoranes stabilised by acyl groups have been prepared quantitatively from the reaction of tertiary phosphines (2 mol.) and diacylacetylenes (1 mol.). The phosphoranes are thermally less stable and more readily hydrolysed than the corresponding phosphoranes stabilised by ester groups. The hydrolyses occur with expulsion of tertiary phosphine and the driving force for the fragmentation is shown not to be derived from the donation of electrons by a phosphoranyl group but probably from the formation of a PO bond. The diphosphorane derived from diphenylmethylphosphine and dimethyl acetylenedicarboxylate has been synthesised by an alternative route.