An experiment is carried out in the present study to investigate the characteristics of the frictional pressure drops for the evaporation of refrigerants R-134a and R-407C in horizontal small tubes having the same inside diameter of 0.83 mm or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m(2) s, imposed heat flux q from 5 to 15 kW/m(2), inlet vapor quality x(in) from 0.2 to 0.8, and refrigerant saturation temperature T-sat from 5 to 15 degrees C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m(2) s with the other parameters varied in the same ranges as those for D-i = 2.0 mm. In this study, the effects of the inlet refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured frictional pressure drops are examined in detail. Our experimental data clearly show that both the R-134a and R-407C frictional pressure drops increase significantly with the inlet vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the effect of the imposed heat flux on the frictional pressure drop is rather weak. Moreover, a significant decrease in the frictional pressure drop results for a rise in T-sat. Furthermore, both the R-134a and R-407C frictional pressure drops increase substantially with the refrigerant mass flux. We also note that under the same x(in), T-sat, q and D-i, refrigerant R-407C has a lower frictional pressure drop when compared with that for R-134a. For the same refrigerant, a reduction in the duct size from 2.0-mm to 0.83-mm causes a significant increase in Delta P-f. Finally, an empirical correlation for the friction factor for the R-134a and R-407C evaporation in the 0.83-mm and 2.0-mm small tubes is proposed. (C) 2007 Elsevier Ltd. All rights reserved.