A crucial chain-branching step in autoignition is the decomposition of ketohydroperoxides (KHP) to form an oxy radical and OH. Other pathways compete with chain-branching, such as “Korcek” dissociation of γ-KHP to a carbonyl and an acid. Here we characterize the formation of a γ-KHP and its decomposition to formic acid + acetone products from observations of n ‑butane oxidation in two complementary experiments. In jet-stirred reactor measurements, KHP is observed above 590 K. The KHP concentration decreases with increasing temperature, whereas formic acid and acetone products increase. Observation of characteristic isotopologs acetone‑ d 3 and formic acid- d 0 in the oxidation of CH 3 CD 2 CD 2 CH 3 is consistent with a Korcek mechanism. In laser-initiated oxidation experiments of n -butane, formic acid and acetone are produced on the timescale of KHP removal. Modelling the time-resolved production of formic acid provides an estimated upper limit of 2 s ‑1 for the rate coefficient of KHP decomposition to formic acid + acetone.