The effect of ring substitution on the kinetics of reaction of arenes, heterocycles, and alkenes with hydroxyl radical is investigated in terms of reactivity and selectivity, using laser flash photolysis (LFP) in acetonitrile solution. The LFP data indicate that charge-transfer contributions in the transition state play an important role in dictating reactivity, and there is a correlation between the experimental and calculated ionization potentials of the arenes and alkenes and their respective reactivities. The reactivity observed for arenes in acetonitrile exhibits a much greater sensitivity toward substitution on the ring than in water, and therefore aqueous data cannot be used to predict reactivity in nonaqueous environments. Nonaqueous solution data may be predictable from gas phase data, and vice versa.