A new model that predicts the reversible ac voltammetric peak profile of a surface redox reaction for an arbitrary choice of the ac voltage amplitude is described. The model is termed the Nernstian Electron Source (NES) model since it is based on a superposition of the fluctuating ac voltage onto a Nernstian distribution of states. The model extends previous theoretical treatments of ac voltammetry which were based on equivalent circuit models that are strictly valid only for small voltage perturbations. Two ferrocene-based monolayer systems were studied to test the predictions of the new model. The dependence of peak height on voltage amplitude for one such monolayer system deviated from the predictions of the older equivalent circuit model at modest amplitudes (E-ac<25 mV) but was in excellent agreement with the predictions from the NES model at all measured amplitudes. The use of large-amplitude ac voltammetry for increasing signal amplitude and for driving otherwise slow surface redox reactions at appreciable rates is also demonstrated.