The electrolyte system of LiBF4 in propylene carbonate (PC) + diethyl carbonate (DEC) was measured for its electrolytic conductivity K at salt molalities m, solvent mass fractions w, and temperatures theta in the ranges of (0.2 to 2.1) mol kg(-1) for m, (0 to 0.7) for w of DEC and (-80 to 60) degreesC for theta, and for its glass transition temperatures T, in the same ranges of m and w. The measured K(M,w) data at different theta were further fitted with an extended version of the Casteel-Amis equation in order to correlate the changes of kappa with simultaneous changes of m and w and with theta. The K surfaces according to these fitted equations all assumed a "dome" shape as a result of kappa peaking in both m and w. Furthermore, as theta was decreased, these domes reduced in height and shifted in the direction of low m and high w, the direction of lower viscosity eta. The T, was found to increase with increasing m and decreasing w, indicating a concurrent change in the eta of the solution. These results are in complete qualitative agreement with those of LiPF6 in PC + DEC. Quantitatively, however, the T-g of the LiBF4 solution is lower than that of the LiPF6 solution, indicating a lower q of the former, and the kappa is generally lower because of the stronger ion association of Li+ with BF4- than with PF6-, except at high m and low theta, where the LiBF4 solution becomes more conductive because of its lower q.