The nematic-to-isotropic phase transitions of 'pure' 4-n-pentyl-4'-cyanobiphenyl (5CB) and nine mixtures of the radical N-(4-n-butyl benzilidine) 4-amino 2,2,6,6-tetramethyl piperidine 1-oxide (BBTMPO) at very low mole fractions in 5CB were investigated by a 'visual' method and the coexistence lines of the phase diagram obtained were found to be virtually linear as quadratic least-squares fits had only slightly higher correlation coefficients than the linear ones. Thus theoretically predicted curvature in the coexistence lines remains unobserved experimentally. The result for one of the mixtures was confirmed by two methods: the density approach and an EPR method that made use of a setup which allows the control of the temperature of the sample to +/-0.01 degreesC. In the EPR study BBTMPO acted as both probe and impurity, thereby eliminating the effect of the addition of a probe to the mixture that is being investigated. The agreement between the results of the EPR, density and visual measurements was very good. The study shows that BBTMPO is a suitable EPR probe and that a nitroxide probe need not be deuterated for investigating the two-phase region of an impure liquid crystal. (C) 2004 Elsevier B.V. All rights reserved.