The conductivity of quaternary ammonium tetrafluoroborates having sizes between those of tetramethylammonium and tetraethylammonium has been measured in propylene carbonate at 25-degrees-C. The limiting molar conductivities and the ion association constants of these electrolytes were calculated by the Shedlovsky extrapolation method. The single ion limiting molar conductivities of quaternary ammonium ions decreased in the order : Me4N+, 14.7 > Me3EtN+, 14.5 > Me2Et2N+, 14.2 > MeEt3N+, 14.0 > Et4N+, 14.4 S cm2 mol-1. The ion association constants decreased from 3.7 to 1.6 dm3 mol-1 with increasing the cation size.