The protonation and hydration of carbon suboxide (O=C=C=C=O) were studied by ab initio molecular orbital methods. While the geometries of the stationary points were optimized using MP2/6-31G(d,p) calculations, relative energies were estimated using QCISD(T)/6-31G(d,p) and 6-311++G(d,p)+ZPE. The behaviour of carbon suboxide was compared with that of carbon dioxide and ketene. The protonation at the beta-carbon is consistently favoured over that at the oxygen; the proton affinities (PA) are estimated to be PA(C3O2) = 775 +/- 15 and PA(H2CCO) = 820 +/- 10 kJ mol(-1) (experimental: 817 +/- 3 kJ mol(-1)). The PAs at oxygen amount to 654, 641 and 542 kJ mol(-1) (experimental: 548 kJ mol(-1)) for C3O2, H2CCO and CO2, respectively. Using the approach of one and two water molecules to model the hydration reaction, the calculated results consistently show that the addition of water across the C=O bond of ketene, giving a 1,1-ethenediol intermediate, is favoured over the C=C addition giving directly a carboxylic acid. A reverse situation occurs in carbon suboxide. In the latter, the energy barrier of the C-C addition is about 31 kJ mol(-1) smaller than that of C=O addition. The C=C addition in C3O2 is inherently favoured owing to a smaller energetic cost for the molecular distortion at the transition state, and a higher thermodynamic stability of the acid product. Molecular deformation of carbon suboxide is in fact a fairly facile process. A similar trend was observed for the addition of H-2, HF and HCl on C3O2. In all three cases, the C=C addition is favoured, HCl having the lowest energy barrier amongst them. These preferential reaction mechanisms could be rationalized in terms of Fukui functions for both nucleophilic and electrophilic attacks, Copyright (C) 2000 John Wiley & Sons, Ltd.