The formation of cyclopentenyl cation from (i) dissociation of ionized cyclohexene, and, (ii) ion-molecule reaction between ionized 1,3-butadiene and ethylene is confirmed by means of the corresponding thermochemistry. Measurement of the Gibbs free energy and enthalpy for deprotonation of cyclopentenyl cation is provided by Fourier transform ion cyclotron resonance experiments and use of the thermokinetic method. The values (GB = 795.0 +/- 1.2 and PA = 824.9 +/- 1.1 kJ/mol, respectively) allow the determination of the heat of formation of the cyclopentenyl cation to be 840.6 +/- 4.1 kJ/mol in good agreement with existing data. (C) 2002 Elsevier Science B.V. All rights reserved.