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The diffusion-controlled one-electron wave (wave I) due to cyclohex-2-en-1one (R) electroreduction on mercury from aqueous solutions in the range 2.5 < pH < 4 is shown to be consistent with the following mechanism : R + HA+ RH+ +A-where HA is a proton donor and the rate-determining coupling step (1 c), denoted by rds, proceeds in the adsorbed state. Double-potential-step chronocoulometric measurements indicate that R adsorption lies below the limits of sensitivity of the method, whereas R2H2 adsorption is appreciable at all potentials positive to -1.4 V us. SCE. The limiting current of wave I in the range 4 < pH < 6 is controlled by the protonation step (1 b), which is shown to proceed both heterogeneously and homogeneously. The contribution of the various proton donors, including adsorbed water, is pointed out. The diffusion-controlled one-electron wave (wave 11), which develops 4-2

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