By means of electrochemical impedance spectroscopy the relative dielectric constant was measured as a function of composition and temperature in the butyronitrile-chloroethane system from -35 to -105 degrees C. A customized cell was designed by iterative optimization; the equivalent circuit was used to assess the impacts of the electrical properties of the sample, the limitations of the instrumentation, and the data reduction technique. To account for strong local ordering effects due to molecular association in these solutions, a new model, termed "extended Kirkwood-Onsager" (EKO), was developed. For solutions rich in chloroethane, structural features are inferred with this model.