The reversible proton dissociation and geminate recombination of photoacids is studied as a function of pressure in liquid ethanol. For this purpose, we used a strong photoacid, 5,8-dicyano-2-naphthol (DCN2) (pK(a)* approximate to -4.5 in water), capable of transferring a proton to alcohols. The time-resolved experimental data are explained by the reversible diffusion-influenced chemical reaction model. At low pressure, the proton-transfer rate increases with pressure, while at high pressure, the rate constant decreases as the pressure increases. The pressure dependence is explained using an approximate stepwise two-coordinate proton-transfer model. The model is compared with the Landau-Zener curve-crossing proton-tunneling formulation. Decrease of the proton-transfer rate at high pressures denotes the adiabatic limit, while the increase in rate at low pressures denotes the nonadiabatic limit.