In sum, we report the first concise reaction mechanism of the transition metal free in situ formation of benzaldehyde under ambient conditions. Our combined experimental and computational studies unambiguously showed that dioxygen and water are essential for this complex radical process. We could show that KO t Bu acts solely as a deprotonating agent, that the concentration of O2 is critical for a rapid conversion and that the presence of a transition metal catalyst is not required. The overall mechanism can be decomposed into two separate HAT reactions that eventually lead to oxidation of benzyl alcohol to benzaldehyde. The use of EPR spectroscopy was able to confirm the presence of a radical intermediate and corroborate theoretical calculations. The forgoing of expensive transitionmetal catalysts in the formation of aldehydes from alcohols is highly desirable. Thus, the detailed mechanism provides starting points for future investigation and optimization of this and similar systems.