Rate consts. for the reaction OH + HCH -> products over an extended range of temps. and pressures were measured using a flash photolysis technique combined with resonance absorption for the detection of OH. The const. k1 increases with pressure at 20-500 mbar (N2 diluent) but shows little temp. dependence at 298-500 K. At the high-pressure limit, k1 can be represented by k1...(298-500 K) = (1.2 +- 0.5)[10-13 exp(-400/T) cm3/mol.s]. Above 500 K, a substantial increase in the apparent activation energy is obsd. The change of Arrhenius parameters is attributed to a change of mechanism from OH addn. at low temps. to dominant H abstraction or exchange at elevated temps.