As the potential applied to the bromide on Ag(001) thin layer electrochemical cell increases past a critical level, the bromide adlayer undergoes a second order phase transition from a disordered state to an ordered state. Using surface X-ray diffraction we measured the time response of this phase transition due to an applied step potential. We find that the time response of the phase transition is limited by the properties of the thin layer geometry. By modeling the electrochemical cell as a RC circuit, we argue that the observed time delay is primarily due to the slow diffusion of charge into the central region of the electrode surface. Crown Copyright