A new model is presented for the analysis of hydrodynamics and heat transfer which can be used to predict heat transfer coefficients in horizontal slug flow. The results of Jung [Horizontal-flow boiling heat transfer using refrigerant mixtures, EPRI Rep., 1989, ER-6364] show a considerable variation of the heat transfer coefficient around the periphery of the tube at low qualities, with the top of the tube having the highest coefficient. It is shown that this region is likely to be one in which slug flow occurs. The transition into the low quality regime is shown to be closely associated with that into slug flow, using an existing flow pattern map. The model follows the traditional interpretation of evaporative heat transfer, namely that of forced convective heat transfer which may be enhanced by bubble nucleation processes. Slug flow hydrodynamic parameters such as liquid slug and inter-slug ("film") lengths to total slug unit length, and the top and bottom film thickness and liquid film velocities in the inter-slug regions are important in understanding the heat transfer phenomena. Therefore, heat transfer models developed for pipe top and bottom should include the effects of these slug flow parameters. (C) 2003 Elsevier Ltd. All rights reserved.