An experimental investigation is performed to study the control of dissolved gases and their effect on heat transfer and pressure drop during the flow of water in a microchannel. An apparatus is developed to deliver water with different levels of dissolved air for use in heat transfer experiments. Six parallel microchannels, each having a hydraulic diameter of 207 mum, are fabricated in copper. During the flow boiling studies with water in these microchannels, nucleation was observed at a surface temperature of 90.5degreesC for the dissolved oxygen content of 8.0 parts per million (ppm) at a pressure of 1 atm with untreated de-ionized water. For the dissolved oxygen contents of 5.4 and 1.8 ppm, nucleation is not observed until the surface temperature reached 100degreesC at a pressure of 1 atm. A slight reduction in heat transfer is noted as the bubbles begin to nucleate in the 8.0 ppm case due to the formation of an insulating bubble layer on the heater surface. Previous investigators for flow boiling in large diameter channels did not observe such behavior. Further downstream, the heat transfer is observed to increase due to bubble activity. This result is in agreement with previous studies. (C) 2003 Elsevier Ltd. All rights reserved.