The conductivity kappa of propylene carbonate-diethyl carbonate (PC-DEC) and PC-EC (ethylene carbonate) solutions of LiBF4 was experimentally determined at temperatures theta from 60 to -80degreesC and at salt molalities m from 0.2 to 2.1 mol kg(-1) and solvent compositions w from 0 to 0.7 weight fraction of DEC for the former solution and from 0.3 to 2.7 of m and 0 to 0.6 of w of EC for the latter. Viscosity eta of these solutions was also studied by measuring their glass transition temperatures T-g in the same ranges of m and w. The T-g was found to rise with m, and fall with w of DEC but rise with w of EC, indicating a concurrent change in the eta of the solutions. The kappa of the PC-DEC solution of LiBF4 peaked in both m and w thus forming a dome in its 3D presentation in the mw coordinates, while the kappa of the PC-EC solution peaked only in m, resulting in an archshaped surface. As theta lowered, these kappa surfaces fell in height and shifted in the direction of low eta. These observations correlated well with the changes of dielectric constant epsilon and viscosity eta of the solutions with the same set of variables. A more detailed study of the effects of DEC and EC on the kappa of PC solution of LiFB4 demonstrated that ion association in these solutions was very weakly dependent on theta, in agreement with an earlier finding. The kappa-T data of the PC-DEC solution was fitted with a Vogel-Fulcher-Tammann equation for an evaluation of its vanishing mobility temperature T-0 and apparent activation energy, E-a, both were shown to form simple surfaces in the mw coordinates slanting up in the direction of high eta. Furthermore, when compared to the T-g surface of the same solution, the T-0 surface was oriented similarly but lower in value by a few dozen degrees. (C) 2003 The Electrochemical Society.