A nonlinear dynamical analysis is made for the generation of potential spikes in response to the change in the current in the electrochemical oxidation of formic acid on Pt, based on a mechanistic model. This model assumes the potential-dependent adsorption, desorption, and reaction of the adsorbed water and carbon monoxide. The model is found to reproduce the spikes observed experimentally in a previous report. The trajectory of the state point in the state space shows how and why the system generates spikes.