Aims: The objective of this study was to determine the effect of increasing juice soluble solids above 40 degrees Brix on wine yeast's ability to grow and ferment the juice, with particular focus on acetic acid production, titratable acidity (TA) changes and the maximum amount of sugar consumed by the yeast. Methods and Results: Riesling Icewine juices at 40, 42, 44 and 46 degrees Brix were inoculated with K1-V1116 at 0.5 g l(-1) and fermented at 17 degrees C until sugar consumption ceased. Increasing soluble solids showed strong negative linear correlations with yeast growth, sugar consumption and ethanol production (r = -0.999, -0.997 and -0.984, P < 0.001, respectively). Acetic acid, glycerol and TA production normalized to sugar consumed showed strong positive correlations to the initial juice concentration (r = 0.992, 0.963 and 0.937, P < 0.001, respectively) but no correlation was found for ethanol production. The acetic acid produced as a function of sugar consumed was positively correlated to the glycerol produced (r = 0.970, P < 0.001). The final TA of the wines ranged between 11.8 and 13.7 g l(-1) tartaric acid, increasing by 2.3-3 g l(-1) over the starting juice. The increase in TA was positively correlated to the increase in acetic acid produced after normalizing the data to the amount of sugar consumed (r = 0.975, P < 0.001). The acid equivalents resulting from the increase in acetic acid accounted for 80-100% of the TA increase when converted to units of tartaric acid. In the final Icewines, acetic acid represented 19-20% of wine TA. Conclusions: Increasing Icewine juice concentration from 40 to 46 degrees Brix increases the proportion of yeast sugar metabolism towards glycerol and acetic acid production to cope with the increased osmotic stress by decreasing yeast growth, sugar consumption rate, the total amount of sugar consumed and the total amount of ethanol produced. The high proportional contribution of acetic acid to titratable acidity in Riesling Icewine may affect acidity perception. Significance and Impact of the Study: We have determined that 10% v/v ethanol would not be achievable with initial juice concentrations above 42 degrees Brix and that Riesling Icewine juice above 52.5 degrees Brix would be theoretically unfermentable. The high proportional contribution of acetic acid to TA may be an important factor in the organoleptic balance of these Icewines.