When a methyl ketone is added to the Belousov-Zhabotinskii (BZ) reaction, an increase of the induction period and in the number of oscillations is observed. This behavior can be explained by the competition of malonic acid and the methyl ketone for molecular bromine. We studied the effect of four methyl ketones in the BZ reaction and found a direct relationship between the induction period, the concentration, and the enolization constant of the ketone. A discussion of the results with regard to the crucial concentration of bromomalonic acid is given. The experimental results are supported by simulations based upon the GTF mechanism of the reaction.