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<p>Although secondary organic aerosol (SOA) is a major component of PM<span class="inline-formula">_2.5</span> and organic aerosol (OA) particles and therefore profoundly influences air quality, climate forcing, and human health, the mechanism of SOA formation via Criegee chemistry is poorly understood. Herein, we perform high-level theoretical calculations to study the gas-phase reaction mechanism and kinetics of four Criegee intermediate (CI) reactions with four hydroxyalkyl hydroperoxides (HHPs) for the first time. The calculated results show that the consecutive reactions of CIs with HHPs are both thermochemically and kinetically favored, and the oligomers contain CIs as chain units. The addition of an <span class="inline-formula">−OOH</span> group in HHPs to the central carbon atom of CIs is identified as the most energetically favorable channel, with a barrier height strongly dependent on both CI substituent number (one or two) and position (<i>syn</i>- or <i>anti</i>-). In particular, the introduction of a methyl group into the <i>anti</i>-position significantly increases the rate coefficient, and a dramatic decrease is observed when the methyl group is introduced into the <i>syn</i>-position. These findings are expected to broaden the reactivity profile and deepen our understanding of atmospheric SOA formation processes.</p>