The effect of hydrostatic pressure on the 1-e(-) reductions of acetophenone (AP) and nitrobenzene (NB) in the absence of an inert solvent is reported. Steady-state voltammetric responses for these redox systems are obtained at a 12.5 mu m radius Pt microdisk in a three-electrode, high-pressure electrochemical cell. Reduction of 8.0 M AP and 9.1 M NB in solutions containing only a small quantity of supporting electrolyte (0.2 M [(n-butyl)(4)N]PF6) yields well-defined sigmoidal-shaped voltammetric waves over the range of pressures investigated (P = 1-1000 bar). Transport-limited currents for both AP and NB reduction are shown to decrease linearly with increasing pressure. Apparent (or integral) diffusion coefficients (D-app) are computed from the voltammetric limiting currents for AP and NB reduction and compared to bulk solution molecular diffusivities (D) measured for a redox-active molecule (decamethylferrocene) added to the solutions at low concentration (similar to 16 mM). The relative decreases in D-app and D with increasing pressure are essentially identical in both NB and AP solutions for pressures up to 1000 bar. The results indicate that transport-limited currents for the reduction of these organic liquids are determined solely by diffusive processes.