Formation and decay of formaldehyde oxides (CH₂OO) affect the complete oxidation of formaldehyde. However, the speciation and reactivity of CH₂OO are poorly understood because of its extremely fast kinetics and indirect measurements. Herein, three isomers of CH₂OO (i.e., main formic acid, small dioxirane, and minor CH₂OO Criegee) were in situ determined and confirmed as primary intermediates of the room-temperature catalytic oxidation of formaldehyde with two reference catalysts, that is, TiO₂/MnOₓ–CeO₂ and Pt/MnOₓ–CeO₂. CH₂OO Criegee is quite reactive, whereas formic acid and dioxirane have longer lifetimes. The production, stabilization, and removal of the three intermediates are preferentially performed at high humidity, matching well with the decay rate of CH₂OO at approximately 6.6 × 10³ s⁻¹ in humid feed gas faster than 4.0 × 10³ s⁻¹ in dry feed. By contrast, given that a thinner water/TiO₂ interface was well-defined in TiO₂/MnOₓ–CeO₂, fewer reductions in the active sites and catalytic activity were found when humidity was decreased. Furthermore, lethal intermediates mostly captured at the TiO₂/MnOₓ–CeO₂ surface suppressed the toxic off-gas emissions. This study provides practical insights into the rational design and selectivity enhancement of a reliable catalytic process for indoor air purification under unfavorable ambient conditions. Copyright © 2019 American Chemical Society.