Here we report field-effect modulation of solution electrochemistry at 5 nm thick ZnO working electrodes prepared on SiO2/degenerately doped Si gates. We find that ultrathin ZnO behaves like a 2D semiconductor, in which charge carriers electrostatically induced by the back gate lead to band edge shift at the front electrode/electrolyte interface. This, in turn, manipulates the charge transfer kinetics on the electrode at a given electrode potential. Experimental results and the proposed model indicate that band edge alignment can be effectively modulated by 0.1-0.4 eV depending on the density of states in the semiconductor and the capacitance of the gate/dielectric stack.