Liquid-liquid equilibria (LLE) for the quaternary system water + tetrahydrofuran + toluene + 1-butanol mixture were measured at 25 degrees C and atmospheric pressure. Binodal curves, tie-lines, distribution, and selectivity for the quaternary system have been determined to investigate the effect of using binary solvents, toluene, and I-butanol, on extracting tetrahydrofuran from aqueous solution. In addition, these experimental tie-line data were also compared with the values predicted by the UNIFAC model. It is found that the UNIFAC group interaction parameters used for LLE could not provide a good prediction.