Quantum chemical calculations at the DFT (B3LYP) and ab initio level (CCSD(T)) have been carried out for the transition states and reaction products of the addition reactions of H2, NH3, CH4, H2O, C6H6, C2H6, C2H4, C2H2, CH3Cl, CH3F and SiH4 to model N-heterocyclic carbenes (NHCs) and P-heterocyclic carbenes (PHCs). The calculations show that PHCs have substantially lower activation barriers than NHCs for breaking the single bonds H–H, O–H, N–H, C–H, C–F, C–Cl and Si–H, as well as the π-bonds in benzene, ethylene and acetylene. The main reason for the higher reactivity of PHCs is their energetically lower-lying LUMO compared to NHCs. The energy level of the LUMO and the electrophilicity of PHCs strongly depends on pyramidalization at the carbene centre. Bulky ligands stabilize intrinsically unstable PHCs because they enforce a more planar arrangement at the carbene centre, which enhances the π-donation from the phosphorus lone-pair MO to the formally empty p(π) orbital at the divalent carbon atom. This raises the energy level of the LUMO but the higher reactivity of the PHC is preserved.