The decomposition rate of oxalate by hydrogen peroxide has been investigated by a KMnO4 titration method. The rate equation for decomposition of hydrogen peroxide in the aqueous phase is ln([H2O2]/[H2O2](0))=-k(1) . t, where k(1)=0.2, for [H+]<2M, k(1)=0.2+0.34([H+]-2), for [H+]>2M. As the acidity increases over 2M, an acid catalysis effect appeard. The new rate equation proposed for the decomposition of oxalate by hydrogen peroxide is -d/dtX([OX]) = k(2)[H2O2](0)(1-X([OX]))(e(-k1t) [OX](0)/[H2O2](0) X([OX])) The rate constant for decomposition of oxalate, k(2), increased with nitric acid concentration and the effect of hydrogen ion concentration was expressed as k(2)=a[H+](n), where the values for a and n were a=1.54, n=0.3 at [H+]<2M, a=0.31, n=2.5 at [H+]>2M, respectively.