Abstract: VLE data under high vacuum conditions were needed in process technology development of catechol system, in which water, phenol, catechol, hydroquinone were involved. Unfortunately, there were few reliable experiment data under high vacuum conditions, and the reliable data in the dilute condition (the mole fraction less than 0.05) are short. The latter is particular important for separation process design. So a modified UNIFAC model(proposed by Gmehling, 1993) was used to predict the phase behavior of catechol system. To test the reliability of this model, the VLE data of waterphenol and catecholhydroquinone binary systems were measured in a Eill's equilibrium distiller at 20 kPa (absolute pressure) and the interaction parameters of Wilson and NRTL model were fitted by simplex mothod. A comparison between the expermiental values and the correlated results by Wilson and NRTL confirms the reliability of the modified UNIFAC model. So this modified UNIFAC model can be satisfactorily used for process technology development of catechol production and design of separation plants.