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The γ radiolysis of deaerated aqueous solutions containing 10−2 or 10−3M glucose and 10−5 to 10−4M of H2O2 or NO3− has been studied at ∼20° in the applied pressure range 0 to 6.34 kbar. The behavior of H2O2 and NO3− solutions is essentially indistinguishable. In both the H2O2 and NO3− solutions, G(H2) increases with increase in pressure; at 6.34 kbar, G(H2) decreases with increase in H2O2 or NO3− concentration. However, the pressure dependence of G(H2) is not affected by a change in glucose concentration from 10−2 to 10−3M or by a change in atmospheric-pressure pH from ∼7 to 9. The results are interpreted in terms of an effect of pressure on the competition between Reactions [4] and [5], eaq−+NO3−(or H2O2)→no H or H2,eaq−+H2O→H+OH−,with Reaction [5] followed by H+RH→H2+R. Such an interpretation gives a difference in activation volumes of Reactions [4] and [5] as ΔV4‡—ΔV5‡ ≈ 15.7 ml mole−1 and a specific rate k50 ≈ 25M−1·sec−1 at atmospheric pressure. Assumption that Reaction [4] is diffusion controlled permits estimation of ΔV4‡ ≈ 1.6 ml mole−1, from the pressure dependence of water viscosity. Thus ΔV5‡ ≈ −14.1 ml mole−1. The latter value is used to estimate an absolute value of V̄0(eaq−) in the range −1.7 to either 2.7 or 9.6 ml mole−1 (dependent on the model assumed for the activated complex); such a range for V̄0(eaq−) is in satisfactory agreement with a previous estimate.