The effect of pH and associated ionic strength on the primary yields in the radiolysis of pressurised water has been assessed by diffusion-kinetic calculations for temperatures in the range 100–300°C. Account has been taken for ionic strength I up to 0.1 mol kg−1, assuming that the counter ions of H+ in acid solutions and of OH− in base solutions have unit charge. In acid solutions, the H+ ions react with e−aq. The decrease in G(e−aq) and the increase in G(H) with decreasing pH becomes substantial for [H+] ≥ 1 × 10−4 m, but the primary yields of oxidising species are almost constant. In alkaline solutions, the OH− anions affect the spur chemistry of radiation-generated protons and hydroxyl radicals for [OH−] ≥ 1 × 10−4 m. The scavenging of H atoms and hydrogen peroxide becomes significant for [OH−] ≥ 1 × 10−2 m. The total yields G(OH) + G(O−) and G(H2O2) + G(HO2−) are independent of base concentration below 0.01 m. In more alkaline solutions, G(OH) + G(O−) increases, whereas G(H2O2) + G(HO2−) decreases with increasing [OH−]. Calculations showed the substantial yield of the reaction O− + e−aq in 0.1 m base solution. Spur chemistry in alkaline hydrogenated water is not affected by the presence of H2 if less than 0.001 m of hydrogen is added.
