Here we report the first high-pressure investigation of redox processes in surface-confined monolayers. We have explored the electrochemical behavior of ferrocene-terminated self-assembled monolayers (SAMs) on gold electrodes immersed in aqueous solutions containing 1 M NaClO4. Electrochemical measurements conducted at hydrostatic pressures ranging from 0.001 to 6 kbar (1 kbar approximate to 1000 atm) show that the electron-transfer reaction for ferrocene in the monolayer is restricted with the application of pressure, whereas the same reaction for ferrocene in solution is not. The dependence of the cyclic voltammetric peak redox potentials on pressure reveals that the oxidation of the ferrocene within the monolayer becomes thermodynamically and kinetically more difficult at high pressures. At pressures above 1-2 kbar, positive volumes of reaction are associated with the oxidation process, indicating that the oxidation step involves an increase in volume. We also found positive volumes of activation for the oxidation process and concluded that the transition step involves a volume expansion that is coupled with the charge transfer step. A structural transformation that allows for ion complexation upon oxidation of surface-confined ferrocene maybe responsible for the volume increase. Different monolayer samples, exhibiting different voltammetric responses, appear to impose different volume constraints on the charge transfer reaction and, therefore, present different pressure responses within a general common trend. These results point out the importance of structural and environmental effects, via steric constraints, on electron transfer processes in surface-confined monolayer assemblies.