The objective of this study was to investigate the production of SO 2 and SO2 binding compounds by wine yeast and the impact of the production of these compounds on the MLF at various time points during alcoholic fermentation. Fermentations were observed for a number of commercial wine yeasts in a synthetic grape juice and Pinot gris juice and SO 2, acetaldehyde, pyruvic acid and a-ketoglutarate. Measurements were taken at multiple time points during the fermentation. Samples were taken from the fermentations at weekly intervals, sterile ifitered, and inoculated with 0. oeni strain VFO to induce MLF. Significant differences between the amount of SO 2, acetaldehyde and pyruvic acid produced by the various yeast strains were noted. Some yeast strains such as FX10, CK S102, F15 and M69, produced significantly higher SO2 concentrations than other yeast strains and MLF was inhibited in these wines. Insignificant free SO 2 was measured, indicating that bound SO2 rather than free SO 2 was responsible for inhibition. At almost all time points of the alcoholic fermentation, acetaldehyde bound SO 2 was determined to be the dominant species of bound SO2 present, suggesting that MLF inhibition by bound SO 2 was due to acetaldehyde bound SO2.