Ab initio calculations are reported for carboimidophosphene (iminomethylenephosphine), carbodiphos-phene (1,3-diphospha-allene) and their conjugate acids. Lsomerization is favoured by inversion about the C.dbd6;N bond and by rotation about C.dbd6;P bonds and is negligibly small (4.2 kcal mol^{â 1} at DZP//4-31 G) for HP.dbd6;C.dbd6;NH, and large (42 kcal mol^{â 1}) for HP.dbd6;C.dbd6;PH. The electronic structures, harmonic vibrational frequencies, and proton affinities are reported and compared with those of other phospha-cumulenes such as HP.dbd6;C.dbd6;O and H2C.dbd6;C.dbd6;PH. The protonation occurs preferentially at the P atom in HP.dbd6;C.dbd6;NH (1) and the C atom in HP.dbd6;C.dbd6;PH (4). At the MP4SDQ/6-31 G** level, the proton affinities are predicted to be 210 ± 5 for (1) and 193 ± 5 kcal mol^{â 1} for (4)(with zero-point energy corrections). The regiospecificity of the dimerization, (2 + 2) cycloadditions, additions to HX reagents, and reacticns with metals are rationalized or predicted.