A chemical reaction that shows a periodic colour change between magenta and blue with a period of about one minute. It occurs with a mixture of sulphuric acid, potassium bromate(V), cerium sulphate, and propanedioic acid. The colour change is caused by alternating oxidation–reductions in which cerium changes its oxidation state (Ce³⁺ gives a magenta solution while Ce⁴⁺ gives a blue solution). The B–Z reaction is an example of a chemical oscillating reaction – a reaction in which there is a regular periodic change in the concentration of one or more reactants. The mechanism is highly complicated, involving a large number of steps. See brusselator.

The B–Z reaction was discovered by Boris Belousov (1893–1970) in the 1950s and by Anatol Zhabotinskii (1938–2008) in the 1960s.