The adsorption process of salicylic acid (SA) onto chitosan membrane is monitored in real time by the dielectric relaxation spectroscopy (DRS) method. A unique dielectric relaxation, which is related to the macroscale concentration polarization layers (CPLs) in the SA solution caused by the adsorption, is observed. By modeling the measured systems composed of the membrane, the CPLs, and the SA solution, the dielectric spectra are analyzed systematically on the basis of the interfacial polarization theory. The parameters about the constituent phases, i.e., the dielectric constant epsilon(m) and the conductivity kappa(m) of the chitosan membrane, the conductivity distribution (kappa(1) to kappa(2)), and the thickness d(CPL), of the CPL, are obtained. The time-dependent epsilon(m) and kappa(m) give insight into the microstate of the chitosan membrane during the adsorption. Furthermore, the time evolution of the conductivity gradient of the CPL, Delta kappa/d(CPL), is combined to interpret the adsorption mechanism. It is suggested that the noninvasive dielectric monitoring may be applied to many adsorption and release processes.