An apparatus for accurate VLE measurements on ternary cryogenic systems is briefly described. It is a modified version of that introduced and developed by Staveley and co-workers for binary mixtures. The apparatus was tested against published results for CH3F + Xe and N2O + Xe, at 182.33 K, the agreement being much satisfactory for both systems. The mixture CH3F +/- N2O + Xe at the same temperature was selected for the first measurements on a ternary system carried out using the modified experimental arrangement, the operation of which is also summarized. VLE results for 61 ternary points are presented together with the evaluation of the excess molar Gibbs free energy GE for the liquid mixture at that temperature. No ternary azeotrope has been found. G(E) for the three component liquid mixture is not an additive function of the G(ij)(E) for its constituent binary mixtures. For the equimolar (ternary) mixture G(1/3)(E) = (500 +/- 6) J mol(-1), at 182.33 K.