The P analogues of methyldiazonium ion (CH3NN)+ (1), methyldiphosphonium ion (CH3PP)+ (2), methylphosphoazonium ion (CH3NP)+ (3), and methylazophosphonium ion (CH3PN)+ (4) were studied with ab initio techniques. Potential energy surface analysis shows that the C3v-symmetric ions all are minima. 4 corresponds to a shallow minimum along the automerization path of its isomer 3. A bridged structure of 2 exists as a local minimum along the automerization path, and this automerization occurs within a tightly bonded ion-molecule complex with small activation barriers. Methyl cation affinities of 43.0, 71.3, 100.4, and 42.1 kcal/mol were determined for the formations of 1-4, respectively, at MP4(fc,sdtq)/6-31 lG(df,p)//MP2- (full)/6-31 lG(df,p) and including MP2(full)/6-31G(d) vibrational zero-point energies. The thermodynamic stabilities of 2 and 3 with regard to the dissociations (CH3XY)+ → CH3+ + XY might be affected by exothermic XY oligomerization and the reactions (CH3PP)+ → CH3+ + ½P4 and (CH3NP)+ → CH3+ + ⅓D3h(PN)3 were studied. Ions 2 and 3 are predicted to be more stable toward dissociation than 1. These results suggest that 2 and 3 should be detectable in the gas phase and possibly also in superacidic media. Like its aryl analogue, 3 might be accessible synthetically. Spectroscopic properties including IR spectra, dipole and quadrupole moments, rotational constants, and polarizabilities are reported that may assist in their detection. Electronic structures of 1-4 were studied in a variety of ways. A new energy decomposition method related to the definition of the electron density difference function, Δρ = ρ (CH3XY+) - ρM(CH3+) - ρM(XY), is presented. Charge distributions in 1-4 are discussed with regard to the Lewis notations. CN- and CP-bonded ions clearly show distinct bonding patterns.