The mechanism of the reaction between bromate and iodine in an acidic medium (HClO4), in a closed system, has been investigated by both experimental and computer simulation techniques. The stoichiometry of the reaction is 2BrO(3)(-) + I-2 --> 2IO(3)(-) + Br-2. The reaction is preceded by an induction period whose length is inversely proportional to the concentration of bromate and the square of the acid concentration. The induction period increases upon the addition of iodide and bromide ions, with the effect of bromide ions being less marked. These ions consume HOI and HOBr molecules, which are precursors to the oxidation of iodine. At the end of the induction period iodine is suddenly depleted while simultaneously a transient interhalogen, iodine bromide, IBr, is formed and consumed rapidly. As soon as the IBr concentration reaches its maximum value, i.e., [IBr](max) = 2[I-2](0), if is rapidly consumed at an exponential rate given by -d[IBr]/dt = k(1)[IBr]. When all the IBr has been depleted, molecular bromine is formed at the rate d[Br-2]/dt = k(2)[H+][BrO3-][I-2]. Values of k(1) and k(2) were evaluated as 0.47 +/- 0.10 s(-1) and 0.26 +/- 0.02 M(-2) s(-1), respectively. A 17-step mechanism which encompasses the mechanisms of the bromate-iodine and bromate-iodide reactions gives good agreement between experimental data and computer simulation. An extensive set of experimental data is presented that supports a molecular mechanism over a radical-dominated one.