We introduce both theoretically and experimentally a new electrochemical technique based on measuring the fluctuations of the faradaic current during redox cycling. By analogy with fluorescence correlation spectroscopy (FCS), we refer to this technique as electrochemical correlation spectroscopy (ECS). We first derive an analytical expression of the power spectral density for the fluctuations in a thin-layer-cell geometry. We then show agreement with measurements using ferrocenedimethanol, Fc(MeOH)2, in water and in acetonitrile in microfabricated thin-layer cells with a ∼70 nm electrode spacing. The fluctuation spectra provide detailed information about the adsorption dynamics of Fc(MeOH)2, which cause an apparent slowing of Brownian motion. We furthermore observe high-frequency fluctuations from which we estimate the rates of adsorption and desorption.