Thermodynamic stability boundary in the structure-H methane + bromocyclopentane mixed hydrate system was measured at pressures from 20 to 100 MPa. The thermodynamic stability boundary of the methane + bromocyclopentane mixed hydrate exhibits anomalous behavior under conditions at high pressures and high temperatures. This phenomenon is due to the elimination and substitution reactions of bromocyclopentane to cyclopentene and cyclopentanol, respectively. The nucleophilic reactions of bromocyclopentane are mainly advanced in the liquid bromocyclopentane-rich phases, while it is restrained when bromocyclopentane is enclathrated in hydrate cage.