Transition metal ions (e.g., Cu2+) catalyze the nonlinear kinetics of thiourea reacting with the following oxidants in batch conditions : [IrCl6](2-), ClO2-, IO3-, BrO3-, and ClO2. Recrystallization of thiourea from distilled water and addition of a chelating agent effectively prevent catalysis by metal ions adventitious in commercial thiourea in the corresponding batch reactions. In continuous-flow experiments, the oscillatory reduction of ClO2- by thiourea shows startlingly little variation in periodic character despite the addition of large amounts of catalytic Cu(TI) or Fe(II) to the flow composition. The present results provide some explanations for the observational and interpretative discrepancies previously reported for these reactions and for the initiation of the ClO2--thiourea reaction in unstirred media by reactor surfaces.