Single crystals of 2-(triphenylphosphoranylidene)succinic anhydride have been X-irradiated and studied by electron paramagnetic resonance/electron nuclear double resonance (EPR/ENDOR) spectroscopy after determination of their crystal structure. The trapped radical results from the departure of a hydrogen atom bound to a carbon located to both a CP and a CO bond. The resulting hyperfine coupling tensors are compared with those obtained from ab initio calculations on H3PCH—H—CHO (1) and are interpreted in terms of spin delocalization on the succinic anhydride ring. The radical stabilization energies have been calculated for H3PCH—H—CHO and H2P—CH2—H—CHO; they agree with the presence of a captodative effect. Owing to the charge separation caused by the contribution of the mesomeric phosphonium–enolate structure, this effect is more pronounced for the phosphoranylidene compound.