The effect of enhanced geometry (pore diameter, gap width) is investigated on the pool boiling of R-123/oil mixture for the enhanced tubes having pores with connecting gaps. Tubes having different pore diameters (and corresponding gap widths) are specially made. Significant heat transfer degradation by oil is observed for the present enhanced tubes. At 5% oil concentration, the degradation is 26-49% for T-sat = 44 degrees C. The degradation increases 50-67% for T-sat = 26.7 degrees C. The heat transfer degradation is significant even with small amount of oil (20-38% degradation at 1% oil concentration for T-max = 44 degrees C), probably due to the accumulation of oil in sub-tunnels. The pore size (or gap width) has a significant effect on the heat transfer degradation. The maximum degradation is observed for d(P) = 0.20 mill tube at T-sat = 4.4 degrees C, and d(p) = 0 23 mm tube at T-sat = 26.7 degrees C. The minimum degradation is observed for d(p) = 0.27 min tube for both saturation temperatures It appears that the oil removal is facilitated for the larger pore diameter (along with larger gap) tube. The highest heat transfer coefficient with oil is obtained for d(p) = 0 23 min tube, which yielded the highest heat transfer coefficient for pure R-123. The optimum tube significantly (more than 3 times) Outperforms the smooth tube even with oil. The heat transfer degradation increases as the heat flux decreases. (C) 2010 Published by Elsevier Ltd.