The transformation of alpha-D-isosaccharinic acid into alpha-D-isosaccharino-1,4-lactone proceeds relatively slowly. Consequently, the transformation constant, K(L), has been determined kinetically in 1.0 mol center dot dm(-3) NaClO(4) and at 23 degrees C. A previous determination in 0.1 mol center dot dm(-3) NaClO(4) and at 23 degrees C has been reinterpreted. The values obtained have been coupled with other data in the literature to demonstrate that the magnitude of the transformation constant is independent of ionic strength, and its value was determined to be log K(L)degrees = 0.80 +/- 0.02. Data from the literature for the dissociation of alpha-D-isosaccharinic acid have been re-evaluated to determine both the "intrinsic" and "composite" dissociation constants at zero ionic strength, namely, log K(u)degrees = -4.04 +/- 0.06 and log K(c)degrees = -4.90 +/- 0.07, respectively. The present data permit a much more thorough understanding of the aqueous chemistry of alpha-D-isosaccharinic acid to be ascertained than has previously been possible.