The reactivity of the prototypical phosphorus-containing ylidion (alpha-distonic ion) (CH2PH3+)-C-. has been investigated in the gas phase by using a dual cell Fourier-transform ion cyclotron resonance mass spectrometer. The ion (CH2PH3+)-C-. and its more stable conventional isomer CH3PH2.+ show distinctly different reactivities toward neutral reagents. This observation contrasts the facile interconversion of the analogous sulfur- and oxygen-containing distonic ions (CH2SH2+)-C-. and (CH2OH2+)-C-. with their conventional isomers CH3OH.+ and CH3SH.+, respectively, within collision complexes in the gas phase. Bracketing experiments yield a proton affinity of 190.4 +/- 3 kcal mol(-1) for the phosphorus atom in (CH2PH2)-C-.. Together with a calculated heat of formation for (CH2PH2)-C-., this value yields a heat of formation of 217 +/- 3 kcal mol(-1) (at 298 K) for the distonic ion (CH2PH3+)-C-..