The thermal and light-induced O − O bond breaking of 2-ethyl-4-nitro-1(2H)-isoquinolinium hydroperoxide (IQOOH) were studied using 1H NMR, steady-state UV/vis spectroscopy, femtosecond UV/vis transient absorption (fs TA) and time-dependent density functional theory (TD DFT) calculations. Thermal O − O bond breaking occurs at room temperature to generate water and the corresponding amide. The rate of this reaction, k = 5.4 · 10−6 s−1, is higher than the analogous rates of simple alkyl and aryl hydroperoxides; however, the rate significantly decreases in the presence of small amounts of methanol. The calculated structure of the transition state suggests that the thermolysis is facilitated by a 1,2 proton shift. The photochemical process yields the same products, as confirmed using NMR and UV/vis spectroscopy. However, the quantum yield for the photolysis is low (Φ = 0.7%). Fs TA studies provide additional detail of the photochemical process and suggest that the S1 state of IQOOH undergoes fast internal conversion to the ground state, and this process competes with the excited-state O − O bond breaking. This result was supported by the fact that the model compound IQOH exhibits similar excited-state decay lifetimes as IQOOH, which is assigned to the S1 → S0 internal conversion. Copyright © 2013 John Wiley & Sons, Ltd.