The reaction of Criegee intermediates with a number of coreactants is examined using theoretical methodologies, combining ROCCSD(T)//M06-2X quantum calculations with theoretical kinetic predictions of the rate coefficients. The reaction of CI with alkenes is found to depend strongly on the substitutions in the reactants, resulting in significant differences in the predicted rate coefficient as a function of the selected alkene and CI. Despite submerged barriers, these entropically disfavored reactions are not expected to affect CI chemistry. The reaction of H2COO + H2COO is found to be barrierless, with a rate coefficient nearing the collision limit, ≥4 × 10(-11) cm(3) molecule(-1) s(-1). The dominant reaction products are expected to be carbonyl compounds and an oxygen molecule, though chemically activated reactions may give rise to a plethora of different (per)acids and carbonyl compounds. CI + CI reactions are expected to be important only in laboratory environments with high CI concentrations. The reaction of H2COO with O3 was predicted to proceed through a pre-reactive complex and a submerged barrier, with a rate coefficient of 1 × 10(-12) cm(3) molecule(-1) s(-1). A study of the dominant CI reactions under experimental and atmospheric conditions shows that the latter reaction might affect CI chemistry.