This paper investigates flow boiling of water in microchannels with a hydraulic diameter of 227 mu m possessing 7.5 mu m wide reentrant cavities on the sidewalls. Average two-phase heat transfer coefficients and CHF conditions have been obtained over a range of effective heat fluxes (28-445 W/cm(2)) and mass velocities (41-302 kg/m(2) s). High Boiling number and Reynolds number have been found to promote convective boiling, while Nucleate Boiling dominated at low Reynolds number and Boiling number. A criterion for the transition between nucleate and convective boiling has been provided. Existing correlations did not provide satisfactory agreement with the heat transfer coefficient but did predict CHF conditions well. (c) 2005 Elsevier Ltd. All rights reserved.