Competition between the reactions[Formula: see text]and[Formula: see text]was measured at 296 K at 1 bar and 5.4 kbar. Values of k5/k3 at 1 bar and (ΔV5≠ − ΔV3≠) averaged between 1 bar and 5.4 kbar for several solutes S are: cyclohexene, 56, + 2.7 cm3/mol; hexene-1, 71, + 2.6 cm3/mol; phenol, 25, −3.8 cm3/mol; benzene, 12, −6.5 cm3/mol. The volume of activation of hydrogen atom addition to the aromatic ring is about 8 cm3/mol more negative than that of addition to a mono-olefin. Rate constants of reaction of solvated electrons with these solutes at 295 K, measured by the pulse-radiolysisspectroscopy technique, are (M−1 s−1): cyclohexene, <1 × 104; hexene-1, 1 × 105; benzene, 4 × 106; phenol, 5 × 107. The phenol reaction with e−solv does not reduce the hydrogen yield by a proportionate amount, so phenol "catalyses" the decomposition of e−solv to form hydrogen, perhaps via C6H5OH−solv → C6H5O−solv + H. The scavenging of hydrogen precursors by hexene-1, benzene, and aniline in n-hexane, reported in ref. 6, had [Formula: see text], −6, and −4 cm3/mol, respectively, all three of which may be attributed to hydrogen atom reactions. The yields of hydrogen from solutions of neohexane in cyclopentane at 1 bar and 5.4 kbar indicate that charge transfer occurs from cyclopentane to neohexane, and that the probability of charge transfer is independent of pressure. The methane yields from these solutions indicate that the decomposition of the neohexane ion formed by charge transfer from cyclopentane, to form methane, is inhibited by pressure.