The electronic structures of methylenephosphorane PH3CH2 and its protonated adduct PH3CH3+ were computed at the SCF level and with inclusion of electron correlation in the IEPA, CEPA, and PNO-CI schemes. For the equilibrium distance Rpc and the force constant kPC weobtained thefollowing values: PH3CH2, RPc(SCF) = 3.16 au (1.67 Å), RPC(CEPA) = 3.19 au (1.69 Å), kPC(SCF) = 5.90 mdyn/Å, and KPC(CEPA) = 5.53 mdyn/Å PH3CH3+, RPC(SCF) = 3.42 au (1.81 \). Rpc(CEPA) = 3.45 au (1.83 \), & PC(SCF) = 3.51 mdyn/\, and KPC(CEPA) = 3.25 mdyn/\. The dipole moment μ(SCF) of PH3CH2 is 3.18 D. The proton affinitiy of PH3CH2 is in the SCF approximation -272.3 kcal/mol and -263.4, -267.9. and -269.7 kcal/mol in IEPA, CEPA, and PNO-CI, respectively The energy of the highest occupied MO agrees reasonably well with the lowest ionization energy of (CH3)3PCH2. The properties of the molecular orbitals and the total electron density were investigated by means of contour diagrams and density plots. In agreement with experimental NMR and previous theoretical results we find the carbanion character in PH3CH2 dominating. Artifacts of the Mulliken population analysis and the arbitra-rjness of defining atomic radii are discussed. From a comparison of RPC and kpc calculated with different basis sets we con-clude that a large amount of the shift in RPC and kPC going from PH3CH3+ to PH3CH2 is due to the carbanion character of PH3CH2 without participation of any d functions. The remaining eitects cannot be simply explained by a dπ-pπ bond in the highest occupied MO only.