Birefringence in a noncubic plastic phase of the two-component system methylchloroform-carbon tetrachloride has been measured in the whole range of mutual concentrations. Analysis of the concentration dependence of birefringence has been carried out using the Vuks model for local polarizing fields. It has been shown that the distortion of molecules contributes significantly to the average molecular polarizability and should be taken into account in the calculation of orientational-order parameters from birefringence.