Solid-liquid equilibrium data were measured for two binary systems of 1,4-butanediol with closely boiling compounds: catechol and 4-methoxyphenol. The experimental results revealed that a 1:1 adducted compound formed in 1,4-butanediol + catechol, whereas 1,4-butanediol + 4-methoxyphenol remained a simple eutectic system. The eutectic loci were further investigated for the mixture of 1,4-butanediol + catechol + 4-methoxyphenol, from which a ternary solid-liquid equilibrium phase diagram was obtained. A feasible separation sequence, on the basis of the phase diagram, was then proposed to separate the eutectic mixture of 4-methoxyphenol + catechol by means of extractive crystallization with 1,4-butanediol as an auxiliary agent. Both the ideal-chemical model and the regular solution-chemical model were applied to correlate the experimental data of the binary systems. The regular solution-chemical model with the determined parameters predicts the equilibrium temperatures of the ternary system to a grand average absolute deviation of 1.24%.