The unimolecular dissociation of energy selected ethylenediamine cations was studied by threshold photoelectron photoion coincidence spectroscopy (TPEPI-CO) in the photon energy range of 8.60-12.50 eV. Modeling the breakdown diagram and time-of-flight distributions with rigid activated complex RRKM theory yielded 0 K appearance energies for eight dissociation channels, leading to NH2CHCH2+center dot at 9.120 +/- 0.010 eV, CH3C(NH2)(2)(+) at 9.200 +/- 0.012 eV, NH2CHCH3+ at 9.34 +/- 0.08 eV, CH2NH2+ at 9.449 +/- 0.025 eV, CH2NH3+ at 9.8 +/- 0.1 eV, c-C2H4NH2+ at 10.1 +/- 0.1 eV, CH3NHCHCH2+ at 10.2 +/- 0.1 eV, and the reappearance of CH2NH2 at 10.2 +/- 0.1 eV. The CBS-QB(3)-calculated pathways highlighted the influence of intramolecular hydrogen attractions on the dissociation processes, presenting novel isomers and low-energy van der Waals intermediates that led to fragments in good agreement with experimental results. While most of the dissociation channels take place through reverse barriers, the 0 K heat of formation of (CH2NH2)-C-center dot was determined to be 147.6 +/- 3.7 kJ mol(-1), in excellent agreement with literature, and the 0 K heat of formation of CH2NH3+ at 844 +/- 10 kJ mol(-1) is the first experimentally measured value available and is in good agreement with theory.