Spontaneous oxidative decarboxylation of alpha-acetolactic acid (ALA) to diacetyl has been assessed under anaerobiosis as a function of pH in water, and in fresh and filtered Lactococcus lactis spp. lactis bv. diacetylactis SD 933 fermented culture media. Whatever the reaction medium, ALA was shown to be potentially reactive, depending on the pH of medium. Diacetyl production mechanism by this strain is discussed on the basis of these kinetic data.