The total absolute rate constant (k(1a) + k(1b)) for the reactions OH + CIO --> Cl + HO2 (k(1a)) --> HCl + O-2 (k(1b)) has been determined in the temperature range from 218 to 298 K at a total pressure of I Torr of helium. Pseudo-first-order conditions were used with ClO in large excess over initial OH. CIO was produced by reacting Cl atoms with an excess Of O-3; both CIO and O-3 were quantitatively determined by UV spectrophotometry between 210 and 310 run. Two sources of OH were used: the reaction of F atoms with excess water vapor and the reaction of H atoms with an excess of NO2. OH was monitored by resonance fluorescence near 308 nm. The absolute rate constant was determined from plots of In [OH] vs time and absolute CIO concentrations. At 298 K, the result is (2.22 +/-0.33) x 10(-11) cm(3) molecule(-1) s(-1). The temperature dependence expressed in Arrhenius form is (7.2 +/- 2.2) x 10(-12) exp[(333 +/- 70)/T] cm(3) molecule(-1) s(-1). The uncertainties are at the 95% confidence limits and include statistical errors from the data analysis and estimates of systematic experimental errors. Numerical simulations show that under the experimental conditions used secondary reactions did not interfere with the measurements. Numerical simulations and experimental checks also show negligible loss of CIO between the time when it is determined by UV absorption and the start of the OH + CIO reaction.