The effect of enhanced geometry (pore diameter, gap width) on convective boiling of R-123/oil mixture was investigated. Three enhanced tube bundles as well as a smooth tube bundle were tested. Test range covered oil concentration, heat flux, mass flux and vapor quality. Significant heat transfer degradation is observed for the present enhanced tubes. Compared with the pool boiling counterpart, however, the heat transfer degradation is much smaller. The two-phase convection in a bundle may remove the oil-rich layer near the heating surface, and reduce the thermal degradation by oil. Of the three enhanced geometries, tube with the largest pore diameter (correspondingly larger gap width) shows the smallest degradation. However, d(p) = 0.23 mm tube, which yielded the highest heat transfer coefficient with pure refrigerant, maintains the highest heat transfer coefficient even with oil. Correlations, which account for the heat transfer degradation with heat flux, oil concentration and enhanced geometry as parameters, were developed. (C) 2011 Elsevier Ltd. All rights reserved.