A simple flow pattern map (vm versus α coordinates) is proposed that provides computational means for selecting steam/water two-phase flow constitutive equations. This flow pattern map can easily be implemented in a thermal-hydraulic code. Determination of the map boundaries between different flow patterns for any channel hydraulic diameter is feasible. This determination is based partially on theoretical criteria that were developed for adiabatic two-phase flows and partially on physical reasoning. Good agreement is obtained between the predicted map regions and experimental data from uniformly heated four-rod bundle and tubes setups.