It has been speculated that the presence of OH (H2O) n clusters in the troposphere could have significant effects on the solar absorption balance and the reactivity of the hydroxyl radical. We have used the G3 and G3B3 model chemistries to model the structures and predict the frequencies of hydroxyl radical/water clusters containing one to five water molecules. The reaction between hydroxyl radical clusters and methane was examined as a function of water cluster size to gain an understanding of how cluster size affects the hydroxyl radical reactivity.