Ah&act-The exact, small-signal impedance and admittance response of a fully dissociated, equi-valent material with arbitrary reaction rates and mobilities of the positive and negative charge carriers is discussed and illustrated for many diRerent parameter values of physical significance. Although such response does not generally lead to an exact equivalent circuit using conventional circuit elements, even including finite-length Warburg diffusion elements, binary data can be fitted to the full binary small-signal response model contained in the available LEVM complex nonlinear least squares fitting program, an approach usually more appropriate than one using approximate equivalent circuits, and a procedure which leads directly to estimates of important microscopic parameters of the system. A useful approach for selecting appropriate starting values of fitting-model parameters is described and illustrated. Detailed fitting of synthetic binary response data containing random errors is illustrated by comparing the results of fits to the true binary model with fits to alternate, approximate equivalent circuit models.