A type of chemical reaction mechanism that leads to an oscillating reaction. It involves the conversion of reactants A and B into products C and D by a series of four steps:

Autocatalysis occurs as in the Lotka–Volterra mechanism and the oregonator. If the concentrations of A and B are maintained constant, the concentrations of X and Y oscillate with time. A graph of the concentration of X against that of Y is a closed loop (the limit cycle of the reaction). The reaction settles down to this limit cycle whatever the initial concentrations of X and Y, i.e. the limit cycle is an attractor for the system. The reaction mechanism is named after the city of Brussels, where the research group that discovered it is based.