The rate coefficient for the O(P-3) + IO reaction has been measured using the discharge-flow kinetic technique coupled to collision-free mass spectrometric detection at 1 Torr nominal pressure (He). IO is generated by the reaction of O with I-2; IO then decays in the presence of excess atomic oxygen. A one-parameter fitting to a numerical model gives a rate coefficient of (1.5 +/- 0.7) x 10(-10) cm(3) molecule(-1) s(-1) at T=298 K. This result is significantly faster than an earlier estimate but is in good agreement with the only previous study of the reaction which employed laser photolysis combined with absorption spectrophotometry. The present result is compared with those for the reactions O(P-3) + XO and X + IO where X = Cl and Br.