In the frame of the JOULE 1 R&D programme of the Commission of the European Communities a project has been carried out on enhanced evaporation heat transfer surfaces. A specific investigation has been realised on refrigerant oil mixtures boiling in a planar confined space. An experimental investigation of the boiling phenomenon in the confined space between a 30 mm wide x 120 mm high, heated plate and an opposing, adiabatic plate was carried out. The heated surface was made of a copper-aluminium-nickel alloy (Ra = 1 micrometer) and a saturated R-113/SUNISO 3GS oil mixture at atmospheric pressure was used as the boiling fluid. The maximum heat flux tested was approximately 90% of CHF. The parameters investigated were the gap size (1-5 mm) and the oil concentration (1-7% by weight). It was again observed that confinement does not improve the nucleate boiling performance of pure R-113 in any significant way, whereas the CHF decreases with decreasing gap size. In addition, while the presence of oil was observed to have only a relatively minor effect on low flux nucleate boiling, it caused a serious degradation of the high flux boiling performance. This deterioration increased with increasing oil concentration and was more severe for smaller gap sizes. However, for a given gap size, the CHF increased with increasing oil concentration accompanied by increasingly larger surface superheats.