The kinetic behavior of OH radicals was studied in the presence of a large excess of n-butane. Hydroxyl radicals were produced by pulsed vacuum-uv photolysis of H2O and were monitored directly by a resonance fluorescence method. The rate constant for the reaction OH + n-butane was determined to be 2.35 (±0.35) × 10-12 cm3 molecule-1 sec-1 at 298 °K.