A three-One flow boiling model is proposed to describe evaporation of elongated bubbles in microchannels. The heat transfer model describes the transient variation in local heat transfer coefficient during the sequential and cyclic passage of (i) a liquid slug, (ii) an evaporating elongated bubble and (iii) a vapor slug. A time-averaged local heat transfer coefficient is thus obtained. The new model illustrates the importance of the strong cyclic variation in the heat transfer coefficient and the strong dependency of heat transfer on the bubble frequency, the minimum liquid film thickness at dryout and the liquid film formation thickness. (C) 2004 Elsevier Ltd. All rights reserved.