The temperature-dependent bimolecular rate constants for the reaction of the hydroxyl radical (HO(•)) with organic matter (OM) (k(OM-HO(•))) have been measured for three natural organic matter (NOM) isolates and three bulk effluent organic matter (EfOM) samples using electron pulse radiolysis and thiocyanate competition kinetics. The range of values for the room temperature k(OM-HO(•)) was 1.21-9.37 × 10(8) M(C)(-1)s(-1), with NOM isolates generally reacting slower than EfOM samples. The NOM isolates had an average apparent activation energy of 19.8 kJ mol(-1), while the EfOM samples had an average value slightly lower (14.3 kJ mol(-1)), although one NOM isolate (Elliot Soil Humic Acid, 29.9 kJ mol(-1)) was a factor of 2 times greater than other samples studied. These apparent activation energies are the first determined for OM and HO(•), and the Arrhenius plots obtained for NOM isolates (lowest R(2) > 0.993) suggest that no significant structural changes are occurring over the temperature range 8-41 °C. In contrast, the greater scatter (lowest R(2) > 0.903) observed for the EfOM samples suggests that some structural changes may be occurring. These results provide a deeper fundamental understanding of the reaction between OM and HO(•) and will be useful in quantifying HO(•) reactions in natural and engineered systems.