Using microfabrication techniques, a microscale platinum heater was fabricated on a Pyrex glass wafer and located in a shallow, but nearly trapezoidal microchannel with a hydraulic diameter of D-h = 56 microns fabricated on another glass wafer. Using a high-speed digital CCD video camera and microscope, the boiling nucleation temperature and two-phase flow patterns were observed and examined at different mass flow rates. The nucleation temperature was found to be reasonably close to the theoretical values as predicted by a 3D numerical heat transfer simulation with the measured bulk temperature of the microheater. The stability of the developed flow indicated three clearly distinguishable two-phase flow regimes: bubbly, wavy and annular. To avoid problems observed in the past, care was taken to ensure that the results were not influenced by the entrance and/or exit regions of the test section. The observed variations in the two-phase flow patterns were compared with the results of a model developed using a stability analysis of the liquid film. (c) 2005 Elsevier Ltd. All rights reserved.