A kinetic study of the degradation and interconversion of glucose and fructose in alkaline solution has been carried out. The change in sugar concentration has been followed by electrometric titration of lactic and saccharinie acids, which are the main products of the degradation in the range of alkali concentration used. A kinetic analysis of the results indicates that all the reactions which occur are of first order with respect to the sugar concentration. In a general discussion of parts I and II of this series, it is shown that the observed dependence of the rates of the degradation and interconversion reactions of glucose upon the sugar and alkali concentration can be explained in terms of the ionic intermediates postulated for the autoxidation reaction. These intermediates are therefore probably concerned in the rate-determining steps of the reactions in the range of alkali concentration used. The absolute velocity constants of most of the reactions have been calculated on this basis. The structure of the intermediates and the mechanism of the reactions are discussed.