This article presents a study of flow boiling of R123 in two small-diameter silver tubes with inner diameters of 1.15 mm and 2.3 mm. The experiments have been accomplished for a wide range of quality variation (0.01-0.9), mass flow rate (650-3000 kg/m(2)s), and heat fluxes (40-80 kW/m(2)). The saturation temperature ranged from 30 to 70 degrees C. In the experiments a peculiar distribution of heat transfer coefficient leading to development of two maxima in its distribution with respect to quality has been observed. Such behavior was seen in both sizes of tubes.