The UNIFAC group contribution model was tested for its ability to correlate and predict the vapour-liquid equilibria (VLB) in the binary systems 1-butylcyclopentene-n-nonane, 1-butylcyclopentene-isopropylbenzene, 1-butylcyclopentene-1-butanol, 1-ethylcyclopentene-n-heptane and 3-ethylcyclopentene-1-butanol. The initial and revised acyclic CC group parameters taken from the literature, and those of the cyclic CC and special cyclopentene groups obtained in this work were used in the computations. In addition, the enthalpies of mixing (HE) of 1-ethylcyclopentene-n-heptane, 1-butylcyclopentene-n-nonane and 1-butylcyclopentene-1-butanol were predicted using the acyclic and cyclic CC group parameters. The model represents well VLE for the hydrocarbon systems, the isothermic vapour pressure and the azeotropic data for those 1-butanols with all sets of group parameters. However, boiling-temperature predictions for the 1-butanol mixtures are considerably poorer when acyclic CC group parameters are used. The prediction of (HE) data is unacceptable although some improvement was obtained by using cyclic CC parameters. New experimental results are reported for the boiling temperatures of the 3-ethylcyclopen-tene-1-butanol mixtures at 200, 400, 600 and 760 Torr. © 1990.