Mutarotation effects during the rapid inversion of sucrose by yeast fructosaccharase are followed over the monomolecular period of hydrolysis. The rotation due to sucrose at time t is added to that of glucose which is calculated from the equation, [image] where ks and k are the monomolecular velocity constants of sucrose inversion and [alpha]-glucose mutarotation resp., P[image], and Po the final and initial ([alpha])D of muta-rotating [alpha]-glucose, and [alpha]/v the initial wt. of sucrose in Ice. The rotation due to fructose can now be found, and the ([alpha])D of this sugar is plotted for various rates of ks against t. Extrapolation of all curves to t = 0, gives an initial ([alpha])D of +15-17[degree] for (2,5) fructose. From these curves and the above equation in which fructose constants are now substituted, k F, the monomolecular velocity constant of the change of (2,5) fructose to equilibriated fructose, is calculated, and is independent of [image]=0.3 between pH 4.6 and 6.1.