The 0 K onset of C3H6 -> C3H5+ + H-center dot is measured by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. From the onset (11.898 +/- 0.025 eV) the heat of formation of the allyl ion (CH2CHCH2+) is determined to be Delta H-f,H-0K degrees = 967.2; Delta H-f.298K degrees = 955.4 +/- 2.5 kJ mol(-1). The value is significantly more positive than prior determinations, and resolves a discrepancy between measurements of the allyl radical and allyl ion heats of formation and recent highly precise measurements of the allyl radical adiabatic ionization energy. The new ally] ion heat of formation leads to a new proton affinity for propadiene (allene) of 765.0 +/- 2.6 kJ mol(-1). An attempt is made to determine the CH3CCH2+ heat of formation by measuring the 0 K onset of 2-ClC3H5 -> C3H5+ + Cl-center dot. However, C3H5+ appears at too low an energy to be the higher energy CH3CCH2+ structure. Rather, 2-ClC3H5+ undergoes a concerted hydrogen transfer and Cl-loss via an intramolecular S(N)2 like mechanism to produce the allyl ion. The 0 K onset of 3-ClC3H5 -> C3H5+ + Cl-center dot (11.108 +/- 0.010 eV) is measured to determine the 3-ClC3H5 heat of formation (Delta(Hf,0K)degrees = 14.9; Delta H-f,H-298K degrees = 1.1 +/- 2.7 kJ mol(-1)). 3-ClC3H5+ is suggested to readily isomerize to trans 1-ClC3H5+ prior to dissociation.