Methyl loss reaction of tert-butylbenzene molecular ion has been studied using mass-analyzed ion kinetic energy spectrometry. Dissociation on a microsecond time scale as investigated by metastable ion decomposition displayed statistical behavior. On the other hand, dissociation of the molecular ion after the absorption of a visible photon was faster than statistically expected. Kinetic energy release in photodissociation was much larger than statistical prediction also, indicating non-RRKM-QET (RRKM = Rice-Ramsperger-Kassel-Marcus; QET = quasi-equilibrium theory) behavior. In the case of collision-induced dissociation which observes reaction of molecular ions excited over relatively broad internal energy range, both the statistical and non-RRKM-QET channels were observed. Two different models are proposed to explain the non-RRKM-QET dissociation from the excited electronic state. These are vibrationally nonergodic processes occurring from the excited or ground electronic states.