We report a very significant cooperative effect of water-ammonia hydrogen bonding in their reactions with a Criegee intermediate, syn-CH3CHOO. Under near ambient conditions, we found that the reaction of syn-CH3CHOO with NH3 becomes much faster (by up to 138 times) at high humidity. Intriguingly, merely adding NH3 (or H2O) alone has almost no effect on the rate of syn-CH3CHOO decay. Quantum chemistry calculation shows that the main reaction involves a hydrogen-atom (or proton) relay in a hydrogen-bonded ring structure; on the product side, a C-N bond is formed and H2O is regenerated as a catalyst. This result demonstrates a new category of reaction in which having two types of hydrogen-bond players (NH3 and H2O) is much more effective than having only one.