The composite kinetic energy release (KER) associated with the decomposition of metastable, protonated fluorobenzene is examined in detail by means of mass spectrometry and analyzed theoretically using a combination of RRKM theory and the statistical adiabatic channel model (SACM). A model for the KER is presented, and contributions of rotational, vibrational, and zero-point energies are calculated. The predictions are in reasonable agreement with the experimentally measured data for two of three components of the KER. With respect to the third, the results lead to a reconsideration of earlier mechanistic models and indicate that ring-opening may occur during the unimolecular decomposition of protonated fluorobenzene. This hypothesis is supported by the ion/molecule reactions of C6H5+ cations formed by dissociative protonation of fluorobenzene with strong Bronsted acids.