The oxidation of isosorbide (S) by chromic acid (CrVI) has been studied in aqueous perchlorate solution at a constant ionic strength of 3.0 mol dm-3 and temperature of 25 C. The course of the reaction was followed spectrophotometrically. The reaction exhibited first-order dependence on [CrVI], less than unit order with respect to [S], and fractional-second-order dependence with respect to [H?]. Variation of the ionic strength or dielectric constant of the medium had no significant effects on the oxidation rate. Addition of MnII inhibited the oxidation rate. The oxidation product of isosorbide was identified as the corresponding monoketone derivative, namely (1S,4S,5R)- 4-hydroxy-2,6-dioxabicyclo[3.3.0] octan-8-one. A mechanism for the oxidation is proposed, and the corresponding rate-law expression has been deduced. The activation parameters associated with the second-order rate constant are presented and discussed