The temperature-dependent kinetics for the reaction of a Criegee intermediate (CH2OO) with propionaldehyde (CH3CH2CHO) was investigated using canonical variational transition state theory (CVT) in conjunction with the small curvature tunneling (SCT) method and the interpolated single point energy (ISPE) method at the CCSD(T)/AUG-cc-pVTZ//B3LYP/6-311G(d,p) level of theory. A rich chemistry was depicted by the title reaction, though the contributions of all of the reaction pathways were limited to atmospheric pressure conditions. The reaction of CH2OO with CH3CH2CHO was identified to proceed via the formation of secondary ozonide (SOZ), which then underwent a sequence of unimolecular isomerization and decomposition reactions to form a variety of products. The obtained rate coefficient for the formation of SOZ at 298 K was determined to be k = 2.44 × 10–12 cm3 molecule–1 s–1. At low temperature, collisionally stabilized SOZ was found to be the more stable product. Contrarily, at high temperature, SOZ degraded to HCHO, and CH3CH2COOH was found to be the major product. The complete degradation mechanism and thermochemistry for the reaction of CH2OO with CH3CH2CHO along with their rate coefficients over the temperature range of 200–1000 K are reported.