C3H52+, formed by electron impact ionization of propane, undergoes metastable decay into C2H2++CH3+. We have monitored this reaction in a magnetic mass spectrometer of reversed geometry that is equipped with two electric sectors (BEE geometry). Three different techniques were applied to identify the fragment ions and determine the kinetic-energy release (KER) of spontaneous Coulomb explosion of C3H52+ in the second and third field free regions of the mass spectrometer. The KER distribution is very narrow, with a width of about 3% [root-mean square standard deviation]. An average KER of 4.58+/-0.15 eV is derived from the distribution. High level ab initio quantum-chemical calculations of the structure and energetics of C3H52+ are reported. The activation barrier of the reverse reaction, CH3++C2H2+ (vinylidene), is computed. The value closely agrees with the experimental average KER, thus indicating that essentially all energy available in the reaction is partitioned into kinetic energy. (C) 2003 American Institute of Physics.