The combined effect of pH, ethanol and fructose on the growth of Saccharomyces cerevisiae at 25°C was studied by the standard Response Surface Methodology. Canonical analysis of the obtained response surface led to the conclusion that the effects of ethanol and fructose can be described by a single factor, the value of water activity, which can be calculated from the other two. Therefore the number of explanatory variables can be reduced. The computational usefulness of the transformation b w = (1 - a w ) is demonstrated.