This paper reports on an experimental study characterizing the laminar-turbulent transition for water flow in circular microtubes. Microtubes with diameters in the range 16.6-32.2 mu m of varying length were employed over the Reynolds number range 300-3400. The volume flowrate was measured for an imposed pressure differential using a timed displacement technique. Additionally, the viscous heating-induced mean fluid temperature rise was measured. Two independent approaches were used to identify transition from laminar to turbulent flow. Both methods showed transition to occur in the Reynolds number range 2100-2500, consistent with macroscale tube flow behavior. (c) 2006 Elsevier Ltd. All rights reserved.