Aims: To verify whether diacetyl can be produced by Lactococcus lactis via amino acid catabolism, and to investigate the impact of the pH on the conversion. Methods and Results: Resting cells of L. lactis were incubated in reaction media at different pH values, containing L-aspartic acid or L-alanine as a substrate. After incubation, the amino acid and metabolites were analysed by HPLC and GC/MS. At pH 5 about 75% of aspartic acid and only 40% of alanine was degraded to pyruvate via a transamination step that requires the presence of alpha-ketoglutarate in the medium, but diacetyl was only produced from aspartic acid. Three per cent of pyruvate was transformed to acetolactate of which 50% was converted into diacetyl. At pH 5.5 and above the pyruvate conversion into acetolactate was less efficient than at pH 5, and acetolactate was mainly decarboxylated to acetoin. Conclusions: Acetoin and diacetyl can be formed as a result of aspartate or alanine catabolism by L. lactis in the presence of alpha-ketoglutarate in the medium. Significance and Impact of the Study: Lactic acid bacteria exhibiting both glutamate dehydrogenase activity and high aspartate aminotransferase activity are expected to be good diacetyl producers during cheese ripening at pH close to 5.