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It has been known for many years that hydrogen atoms can be easily created and studied in water using radiolytic techniques (1). The use of CW EPR detection coupled with electron radiolysis proved extremely useful in estimating many reaction rates, and revealed the interesting phenomenon of chemically induced dynamic electron polarization (CIDEP) (2). In recent years, we have made use of pulsed EPR detection to make precision reaction rate measurements which avoid the complications of CIDEP (3). Activation energies and H/D isotope effects measured in these studies (4-14) will be described below. An interesting aspect of the hydrogen atom reactions is the effect of hydrophobic solvation. EPR evidence--an almost gas-phase hyperfine coupling and extremely narrow linewidth--is quite convincing to show that the H atom is just a minimally perturbed gas phase atom inside a small 'bubble'. In several systems we have found that the hydrophobic free energy of solvation dominates the solvent effect on reaction rates.