The sequential ethene (C2H4) loss channels of energy-selected ethylphosphine ions have been studied using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy in which ion time-of-flight (TOF) distributions are recorded as a function of the photon energy. The ion TOF distributions and breakdown diagrams have been modeled in terms of the statistical RRKM theory for unimolecular reactions, providing 0 K dissociation onsets, E-0, for the ethene loss channels. Three RRKM curves were used to model the five measurements, since two of the reactions differ only by the internal energy of the parent ion. This series of dissociations provides a detailed check of the calculation of the product energy distribution for sequential reactions. From the determined E-0's, the heats of formation of several ethylphosphine neutrals and ions have been determined: Delta H-f degrees(298K)[P(C2H5)(3)] = -152.7 +/- 2.8 kJ/mol, Delta H-f degrees(298K)[P(C2H5)(3)(+)] = 571.6 +/- 4.0 kJ/mol, Delta H-f degrees(298K)[HP(C2H5)(2)] = -89.6 +/- 2.1 kJ/mol, Delta H-f degrees(298K)[HP(C2H5)(2)(+)] = 669.9 +/- 2.5 kJ/mol, Delta H-f degrees(298K)[H2PC2H5] = -36.5 +/- 1.5 kJ/mol, Delta H-f degrees(298K)[H2PC2H5+] = 784.0 +/- 1.9 kJ/mol. These values have been supported by G2 and G3 calculations using isodesmic reactions. Coupled cluster calculations have been used to show that the C2H4 loss channel, which involves a hydrogen transfer step, proceeds without a reverse energy barrier.