Criegee intermediates have implications as key intermediates in atmospheric, organic, and enzymatic reactions. However, their chemistry in aqueous environments is relatively unexplored. Herein, Born-Oppenheimer molecular dynamics (BOMD) simulations examine the dynamic behavior of syn- and anti-CH3 CHOO at the air-water interface. They show that unlike the simplest Criegee intermediate (CH2 OO), both syn- and anti-CH3 CHOO remain inert towards reaction with water. The unexpected high stability of C2 Criegee intermediates is due to the presence of a hydrophobic methyl substituent on the Criegee carbon that lowers the proton transfer ability and inhibits the formation of a pre-reaction complex for the Criegee-water reaction. The simulation of the larger Criegee intermediates, (CH3 )2 COO, syn- and anti-CH2 C(CH3 )C(H)OO on the water droplet surface suggests that strongly hydrophobic substituents determine the reactivity of Criegee intermediates at the air-water interface.