Energy selected trimethyl phosphine ions were prepared by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. This ion dissociates via H, CH3, and CH4 loss, the latter two involving hydrogen transfer steps. The ion time-of-flight distribution and the breakdown diagram are analyzed in terms of the statistical RRKM theory, which includes tunneling. Ab initio and DFT calculations provide the vibrational frequencies required for the RRKM modeling. CH3 loss could produce both the P(CH3)(2)(+) by a simple bond dissociation step, and the more stable HP(CH2)CH3+ ion by a hydrogen transfer step. Quantum chemical calculations are extensively used to uncover the reaction scheme, and they strongly suggest that the latter product is exclusively formed via an isomerization step in the energy range of the experiment. The data analysis, which includes modeling with the trimethyl phosphine thermal energy distribution, provides accurate onset energies for both H (E-0K = 1024.1 ± 3.5 kJ/mol) and CH3 (E-0K = 1024.8 ± 3.5 kJ/mol) loss reactions. From this analysis, we conclude that the &UDelta;H-f(298K)°[HP(CH2)(CH3)(+)] = 783 ± 8 kJ/mol and &UDelta;H-f(298K)°[P(CH2)(CH3)(2)(+)] = 711 ± 8 kJ/mol.