This article is the first in a three part study on flow boiling of refrigerants R236fa and R245fa in a silicon multi-microchannel heat sink. The heat sink was composed of 67 parallel channels, which are 223 mu m wide, 680 pm high and 20 mm long with 80 pm thick fins 2 2 separating the channels. The base heat flux was varied from 3.6 to 221 W/cm(2), the mass velocity from 281 to 1501 kg/m(2) and the exit vapour quality from 2% to 75%. The working pressure and saturation temperature were set nominally at 273 kPa and 25 degrees C, respectively. The present database includes 1217 local heat transfer coefficient measurements, for which three different heat transfer trends were identified, but in most cases the heat transfer coefficient increased with heat flux and was almost independent of vapour quality and mass velocity. Importantly, it was found for apparently the first time that the heat transfer coefficient as a function of vapour quality reaches a maximum at very high heat fluxes and then decreases with further increase of heat flux. (C) 2008 Elsevier Ltd. All rights reserved.