Experiments have been conducted to investigate discrepancies in previously published data for the pressure drop in microchannels. Straight channel test sections with integrated pressure sensors were developed with channel hydraulic diameters ranging from 25 to 100 mu m. Compressible flow results for 6.8 < Re < 18,814 and incompressible flow results for 4.9 < Re < 2068 have been obtained. The results suggest that friction factors for microchannels can be accurately determined from data for standard large channels. The large inconsistencies in previously published data are probably due to instrumentation errors and/or improper accounting for compressibility effects. (c) 2004 Elsevier Ltd. All rights reserved.