The ionization alpha , electron attachment eta and other electron transport coefficients for SF₆ and perfluorocyclobutane (c-C₄F₈) gas mixtures have been calculated by a three-term Boltzmann equation method over the E/N range 283-707 Td. A set of electron collision cross-sections that gives transport parameters in agreement with measurements for the pure gases is used. The calculation is performed for important experimental conditions, i.e. the steady-state Townsend (SST), pulsed Townsend (PT) and time-of-flight (TOF) methods. It is found that the greater the reduction in E/N the more pronounced is the peak of eta values of mixtures, e.g. at E/N=283 Td the peak appears at the SF₆ partial pressure k around 20%, though alpha values show monotonical variation with k, at a fixed E/N for the E/N range studied here. As a result, the E/N value at which alpha = eta , i.e. the limiting E/N of 20% SF₆ and 80% c-C₄F₈, can exceed the values for the respective pure gases. This fact suggests that this combination may be one desirable synergistic mixture since the limiting E/N is an important parameter for predicting the dielectric characteristics in electronegative gases.