This article is the first part of a study on flow boiling of R236fa and R245fa. This part presents pressure drop measurements obtained on a silicon multi-microchannel evaporator with 85μm wide and 560μm high channels separated by 46μm wide fins. The 135 microchannels were 12.7mm long. Dielectric refrigerants R236fa and R245fa were used as the evaporating test fluids. The inlet saturation temperature was maintained at 30.5°C while the mass fluxes were varied from 499 to 1100kg/m2s and the base heat flux was tested from 130 to 1400kW/m2. A new experimental technique was developed to measure the outlet pressure losses, which represented up to 30% of the total pressure drop and thus cannot be neglected. The microchannel pressure drop measurements were very well predicted by the method of Cioncolini et al. (2009). © 2011 Elsevier Inc.