This article presents an experimental study of upward flow boiling heat transfer in multiport tubes. Usually, multiport tubes are utilized horizontally in automotive and air-conditioning systems while here there is interest in vertical flow as part of a two-phase thermosyphon electronics cooling system. Experiments were carried out in a flat aluminum extruded multiport tube, which was composed of 7 parallel rectangular channels (1.1mm × 2.1mm) with a hydraulic diameter of 1.4 mm. Two refrigerants, R245fa and the new refrigerant R1234ze, the latter a new environmentally safe refrigerant proposed as a potential replacement for R134a, were tested. A new hot water heating technique that accounts for either uniform or non-uniform local heat flux distribution along the channel, particularly for exploring high vapor qualities into the dryout region, was developed to obtain and reduce the data. Accordingly, heat transfer coefficients were measured locally for the entire range of vapor qualities starting from subcooled liquid to superheated vapor. Effects of heat flux, mass flux, vapor quality, and saturation temperature on flow boiling heat transfer in multiport tubes were considered. Finally, the experimental results were compared with some well-known correlations to evaluate the capabilities of existing prediction methods. The analysis completed so far shows that the three-zone model for slug flows works well for that subset of test results, utilizing the apparent surface roughness in place of the dryout thickness in the model, as has been previously done for silicon, stainless steel and copper microchannels with measured surface roughnesses. Notably, the non-dryout data were well predicted by the Cooper nucleate pool boiling correlation as in many other studies, although there is no proof that nucleate boiling is taking place except near the point of inception of boiling.