The conductivity and viscosity of pure tributylphosphonium tetrafluoroborate protic ionic liquid (PIL) as well as the mixture with acetonitrile were measured from 20 to 80 degrees C. Those electrolytes exhibit a Newtonian behavior, and the temperature dependence of their viscosity, eta, can be described by the Vogel-Tamman-Fulcher (VET) equation. These transport properties show that the Walden product, eta sigma, is independent of the temperature signifying that disassociation of ions was the dominating factor in combined conductivity and viscosity parameters. The viscosity deviations, Delta eta, for PIL + CH3CN solution, calculated at several temperatures from T = 20 degrees C to T = 80 degrees C, present negative deviation from ideality with minima in the rich-PIL region composition. Finally, thanks to the Debye-Onsager plot, the infinite dilution molar conductivity of the PIL could be determined and thus, the infinite dilution conductance of the cation phosphonium was evaluated. Because of the favorable transport properties (sigma, eta), tributylphosphonium tetrafluoroborate PIL pure and in mixture with acetonitrile appear as promising electrolytes for electrochemical devices.