d-glucuronic acid (GlcA) and d-glucuronolactone (GlcL) were treated with subcritical aqueous ethanol in the range of 0% to 80% (w/w) at 180°C in order to examine the effect of ethanol on the interconversion between GlcA and GlcL. When GlcA was treated at higher ethanol concentrations, less GlcA disappeared and more GlcL was formed compared to treatments at lower ethanol concentrations. For comparison, in the treatment of GlcL at higher ethanol concentrations, disappearance of GlcL was slower and less GlcA was formed. The degradation and interconversion of GlcA and GlcL were kinetically analyzed under the assumptions of first-order kinetics in order to evaluate the rate constants for each process. The rate constants were smaller at higher ethanol concentrations. This was particularly manifested in the significantly low rate constants observed at ethanol concentrations higher than 60%. The treatments of GlcA and GlcL were also examined at 200°C, and the effect of ethanol at 200°C was similar to that at 180°C.