The reaction of OH with ClO has been investigated by ab initio molecular orbital and variational transition state theory calculations. Both singlet and triplet potential energy surfaces predicted by the G2M method are presented. The reaction was shown to take place primarily over the singlet surface by two main channels producing HO2+Cl and HCl+O-2((1)Delta), with the former being dominant. The predicted total rate constant, k(t)=5.27x10(-9) T-1.03 exp (-40/T) cm(3) molecule(-1) s(-1), and product branching ratios in the temperature range 200-500 K at P<200 atm agree satisfactorily with experimental values. The computed branching ratios, k(2)/(k(1)+k(2))=0.073 for HCl+O-1(2) and 0.045-0.048 for DCl+O-1(2) in the temperature range 200-500 K based on the recent experimental heat of formation for HO2 (4.0+/-0.8 kcal/mol) compare closely with the experimental values, 0.07+/-0.03 and 0.05+/-0.02, respectively. At higher temperatures (1000-2500 K), the branching ratios increase slightly to 0.084-0.137 and 0.061-0.111 for the OH and OD reactions, respectively. The rate constant for HO2+Cl and HCl+O-2 production from OH+ClO in the temperature range, 500-2500 K, can be given by k(1)=3.4x10(-13) T-0.3 exp (725/T) and k(2)=5.85x10(-19) T-1.67 exp (1926/T) cm(3) molecule(-1) s(-1), respectively.