While many studies examine the hydrodynamics of two-phase gas-liquid slug flow, the details of the heat transfer mechanism of this flow regime remain largely unknown. Analyzing the detailed hydrodynamics around a single Taylor bubble provides a basis for understanding the heat transfer mechanism. Propagation of a single Taylor bubble is approximately steady; however, in undeveloped slug flow, acceleration and coalescence of consecutive bubbles is unsteady and highly complicated. The present study is aimed to investigate the effects of developing slug flow, particularly in the region where consecutive bubbles coalesce, on the instantaneous heat transfer for different flow regimes and heating locations. To enable controlled flow conditions, experiments were carried out for two consecutive Taylor bubbles rising in a vertical pipe. Measurements of the instantaneous heat transfer coefficient as a function of the leading bubble location were carried out for different separation distances between the two consecutive bubbles. These measurements were performed for various liquid flow rates, corresponding to laminar, transitional and turbulent background flow regimes. The experiments were conducted at several locations corresponding to different heating lengths. Furthermore, a comparison between the heat transfer coefficients of a bubble that has undergone coalescence with those corresponding to a single Taylor bubble with a similar length was performed. For all conditions, it was found that the heat transfer rate is augmented in the presence of an accelerated trailing bubble. The variation of the heat transfer rates is discussed with relation to the local flow hydrodynamics. (C) 2015 Elsevier Ltd. All rights reserved.