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Intube flow boiling experiments for refrigerant R-134a mixed with a lubricating oil are reported. The tests were run at a nominal inlet pressure of 3.4 bar over a wide range of vapor qualities at mass velocities of 100, 200 and 300 kg/m2s (73500, 147000 and 220500 lb/h ft2) for inlet oil concentrations from 0, 0.5, 1.0, 3.0 and 5.0 wt.% oil. At high vapor qulities (x > 0.75-0.85), the local boiling heat transfer coefficients dropped off rapidly with increasing oil concentration. At low to intermediate vapor qualities (0.2 < x < 0.60), oil tended to increase the local boiling coefficient at 300 kg/m2s (220500 lb/h ft2) while significant deterioration in boiling performance occured at the lower two mass velocities. Ther was strong evidence that oil holdup occurred inside the microfin tube test section and was responsible for the sharp falloff in performance at the mass velocity of 100kg/m2s (73500 lb/h ft2), which means there may be a lower mass velocity limit for effective use of microfin tubes.