The model reactions of hydrogen isocyanide plus phosphaethene and its protonated form have been studied by ab initio MO methods, The energy potential surfaces have been explored using MP2/6-31G(d,p) calculations while relative energies between stationary points have been estimated using QCISD(T)/6-31C(d,p) and CISDQ/6-311G(d,p) wave functions in conjunction with zero-point corrections, The HN=C + CH2=PH system is also considered at a higher level by means of the CASPT2/ANO method. With regard to the [2+1] cycloaddition giving phosphiranimine (1), the initial C-attack with an energy barrier of 25 kcal/mol is slightly favored over the P-attack, Two transition structures for [2+2] cycloaddition have also been found, but this approach is much less favored than the [2+1] approach with a difference of at least 30 kcal/mol in the barrier heights. Two four-membered carbene rings resulting from the [2+2] cycloadditions exist; the most stable of the two adducts lies 29 kcal/mol above 1 and is also an intermediate in the rearrangement of 1 to phosphaziridine 2 having an exocyclic C=C double bond (6 kcal/mol above 1), The isomer with an exocyclic C=P bond (3) lies 16 kcal/mol above 1, Overall our results suggest that the [2+1] approach is the dominating mechanism of the HN=C + H2C=PH reaction, The participation of the [2+2] cycloaddition in the transformation can be ruled out. The [2+1] cycloaddition giving 1 is also the rate-determining step, Owing to the fact that the rearrangement 1 --> 2 requires a smaller activation energy and that the energy difference between 1 and 2 is small (6 kcal/mol), 2 could be formed as a minor product, Thus, the reaction differs fundamentally from that involving the isoelectronic HN=C + H2C=SiH2 system where a four-membered carbene has been found to have a high stability, Protonating the phosphorus atom substantially stabilizes the cyclic structures, The four-membered carbene is stabilized by about 9 kcal/mol relative to the neutral structure 1a, Nevertheless, the only thermodynamically favored process in the HN=C + H2C=PH2+ system is a C-nucleophilic addition giving an open nitrilium cation which is by far the most stable protonated species.