The void fractions and frictional pressure drops occurring in two-phase air/liquid flows in smooth-walled, circular-section ducts with diameters from 0.74 to 3.07 mm have been measured for vertical and horizontal flows over a wide range of conditions: 0.05 1000 and it is concluded that fine passage dimensions are do not of themselves present any problems to these correlations under turbulent conditions. For Re L <1000, a modification of the Beattie correlation is recommended. Both the Lockhart-Martinelli and the more complex CISE correlations for void fraction describe the measured void fractions satisfactorily over the whole range of conditions studied