Convective boiling of HCFC123 and FC72 in 0.19, 0.3 and 0.51 mm ID tubes is investigated. The experimental setup as well as the data reduction procedure has carefully been designed, so that the relative uncertainty interval of the measured heat transfer coefficient in microtubes is kept within +/-10%. Up to 70 K liquid superheat over the saturation temperature is observed at low heat and mass fluxes. The onset of the superheat is found to be dependent on the mass flux and the boiling number of the refrigerant examined. In the saturated boiling regime, the heat transfer characteristics are much different from those in conventional-size tubes. The heat transfer coefficient is monotonically decreased with increasing the vapor quality, and becomes independent of the mass flux. Most empirical formulas are not in accordance with the present experimental data. Since the prediction using the nucleate boiling term of Kandlikar's empirical correlations coincides with the present results, the convection effect should be minor in microtubes. On the other hand, the pressure loss characteristics are qualitatively in accordance with the conventional correlation formula while quantitatively much lower. These phenomena can be explained by the fact that the annular flow prevails in microtubes. (C) 2003 Elsevier Ltd. All rights reserved.