Using the relative-rate technique, the rate constant for the reaction of OH radicals with isoprene was determined in one atmosphere of air at 298 +/- 2 K. Methyl nitrite photolysis in air was used as a source of OH radicals. Ozone and O-atom reactions with isoprene were confirmed to have a negligible contribution on the loss of isoprene. The rate constant relative to three reference compounds yielded (10.4 +/- 0.4) x 10(-11) cm(3) molecule(-1) s(-1) (the uncertainty is 2sigma).