Uneven distribution in heat exchangers is a cause of reduction in both thermal and fluid-dynamic performances. Many papers have dealt with single-phase flow and both flow distribution data and analytical or numerical models are available for header design. With regard to two-phase flow, phase separation in manifolds with several outlets is so complicated that, to date, there is no general way to predict the distribution of two-phase mixtures at header-channel junctions. The design of headers for new generation compact heat exchangers and multi-microcliantiel evaporators is still based on all empirical approach, as a number of variables act together: geometrical parameters and orientation of the manifolds and of the channels, operating conditions, fluid physical properties. In the present paper measurements of the two-phase air-water distributions occurring in a cylindrical horizontal header supplying 16 vertical channels are reported for upward flow. The effects of the operating conditions, of the header-channel distribution area ratios and of the inlet port orifice plates were investigated. The flow rates of each phase flowing in the different channels were measured. Time varying, void fraction data were also analysed to characterise the two-phase flow patterns. Video records were taken in order to infer different flow patterns (from intermittent to annular) inside the header-hannel system. (c) 2007 Elsevier Ltd. All rights reserved.