The reactions of OH with molecular chlorine (reaction 1), methane (reaction 2), and propane (reaction 3) have been studied experimentally using, a pulsed laser photolysis/pulsed-laser-induced fluorescence technique C, over wide ranges of temperatures (297-826, 298-1009, and 296-908 K, respectively) and at pressures between 6.68 and 24.15 kPa. The rate coefficients obtained for reactions 1-3 demonstrate no dependence on pressure and exhibit positive temperature dependences that can be represented with modified three-parameter Arrhenius expressions within their corresponding experimental temperature ranges: k(1) = 3.59 x 10(-16) T-1.35 exp(-745 K/T) cm(3) molecule(-1) s(-1), k(2) = 3.82 x 10(-19) T-2.38 exp(-1136 K/T) cm(3) molecule(-1) s(-1), and k(3) = 6.64 x 10(-16) T-1.46 exp(-271 K/T) cm(3) molecule(-1) s(-1). For the OH + Cl-2 reaction, the potential energy surface has been studied using quantum chemical methods, and a transition-state theory model has been developed on the basis of calculations and experimental data. Model predictions suggest OH + Cl-2 --> HOCI + Cl as the main channel of this reaction. The model results in the expression k(1) = 1.35 x 10(-16) T-1.50 exp(-723 K/T) cm(3) molecule(-1) s(-1) for the temperature dependence of the reaction I rate coefficient extrapolation outside the experimental range to low temperatures down to 200 K and to high temperatures up to 3000 K. A temperature dependence of the rate coefficient of the HOCI + Cl --> OH + Cl-2 reaction has been derived on the basis of the experimental data, modeling, and thermochemical information.