AbstractThe Belousov‐Zhabotinskii (BZ) reaction of oxaloacetic acid (OAA) = oxobutanedioic acid is investigated spectrophotometrically both in the closed system and in the continuous‐flow stirred tank reactor (CSTR). Using ferroin as catalyst, the reaction in the closed system exhibits a very complicated oscillatory pattern with three different types of oscillations being separated by induction periods. “Chemical chaos” = deterministic aperiodicity of the type 2 oscillations is observed in a very small range of the ferroin concentrations in the closed system. In the CSTR, change from sustained limit‐cycle oscillations to aperiodicity is produced by small variations of the temperature, or of the ferroin concentration. A qualitative discussion of these results is given in terms of coupled oscillators resulting from the simultaneous BZ reactions of the three equilibrium species of OAA in aqueous solution: the hydrate, the keto and the enol forms. The influence of the stirring rate on the oscillatory pattern in the CSTR is discussed.