A new optical void fraction measurement system has been coupled to a flow boiling test facility to obtain dynamic and time-averaged void fractions in a horizontal tube. A series of evaporation tests have been run for two refrigerants. R-22 was tested under mass velocity conditions of 70, 100, 150 and 200 kg/m(2)s and R-410A for 70, 150, 200 and 300 kg/m(2) s in a 13.6 mm diameter glass tube. Using our newly developed image processing system, about 227000 images have been analyzed in this study to provide the same number of dynamic void fraction measurements. From these images, 238 time-averaged void fraction values have been obtained for vapor qualities from 0.01 to 0.95. These experimental points show very good agreement with the horizontal version of the Rouhani-Axelsson drift flux void fraction model. (C) 2003 Elsevier Ltd. All rights reserved.