The rate of vitamin B 12 loss was 3X greater in heated, chocolate-flavoured milk than in unflavoured milk. Studies of B 12 stability in the presence of cocoa powder components were performed to identify the reactive agents. Cocoa polyphenols with a strong capacity both for reduction and for peroxide generation accelerated B 12 destruction as much as 20-fold vs. polyphenols without both capacities. Polyphenols with both capacities include the cocoa polyphenol (+)-catechin and oligomers thereof, but also the structurally related polyphenols gallic acid, caffeic acid and (À)-epigallocatechin gallate. The demonstrated physical affinity of cocoa powder for B 12 was a significant factor in the destructive process. B 12 may exhibit decreased stability in heated, neutral pH, polyphenol-fortified foods.