The saturation vapour pressure of {x BF3 + (1-x)n-C4H10} has been measured at 195.49 K. The system shows large positive deviations from Raoult's law and liquid-liquid immiscibility over a wide composition range whose limits have been estimated as 0.39+/-0.03 < x > 0.9785. The excess molar Gibbs energies (G(m)(E)) have been calculated M as a function of composition from the vapour pressure data. For the hypothetical equimolar mixture G(m)(E) (x = 0.5) M (703.9 +/- 29) J mol(-1). The results were interpreted using the statistical associating fluid theory for potentials of variable range (SAFT-VR), as well as compared with those of related systems, such as (BF3 + n-pentane) and (BF3 + xenon).