Abstract: The complex kinetics of ClO2―-S2O32－ nonlinear reaction system in an unbuffered solution has been investigated in a CSTR. Both quasiperiodic and mixed-mode oscillations of pH and Pt potential were observed. At relatively low [NaClO2]0/[Na2S2O3]0 ratio, the system changed from small-amplitude oscillations and quasiperiodic oscillations to chaos and pure large-amplitude oscillations with increasing the flow rate. At higher initial concentration ratio, however, quasiperiodicity was given way to mixed-mode oscillations in which each period consists of one large and n small peaks (LSn). LS, LS2,LS3,…, LSn were obtained when the flow rate was decreased. For a narrow range of flow rate between each pair of periodic regions(LSn, LSn＋1), a region of aperiodic behavior, an apparently stochastic mixture of LSn and LSn＋1, was also observed. A general model based upon changes in the oxidation state of sulfur in the presence of a generic oxidant simulated the mixed-mode oscillations and aperiodic behavior.