Extensive experimentation was performed to obtain flow boiling critical heat flux data in single stainless steel microtubes with diameters from 0.286 to 0.700 mm over a wide range of mass fluxes, inlet subcoolings, and exit pressures for two different working fluids (water and R-123). The effect of different operating parameters - mass flux, inlet subcooling, exit quality, heated length and diameter - were assessed in detail (Part I of the paper). The conventional DNB-type behavior is observed in the high subcooled region, and the typical dryout type behavior is seen in the high-quality saturated region when the flow is completely annular. The flow in transitional flow patterns (churn-annular or slug-annular) causes a peculiar increase of CHIF with exit quality. Also, the increased void fraction near the saturated region in subcooled boiling results in increased subcooled CHF values. Part II of the paper deals with comparison of data with existing correlations and development of a new correlation to predict the CHIF condition in the subcooled liquid region. (C) 2009 Elsevier Ltd. All rights reserved.