The effect of the gravitational force has generally been ignored in studies of the hydrodynamics, mass and heat transfer characteristics of Taylor flow in microchannels. In this study, the effects of gravity on the bubble behavior and heat transfer rates for horizontal Taylor flow in millimeter-sized channels were studied experimentally. Significant bubble asymmetry was observed for nitrogen-ethylene glycol Taylor flow in three circular channels of different internal diameters (d=1.12, 1.69, 2.12 mm giving Bond numbers (Bo) of 0.287, 0.653 and 1.028). A parabolic drainage velocity profile inside the liquid film surrounding the bubble was noted through performing Micro-Particle Image Velocimetry (mu PIV) measurements in that region. A simplified model similar to Nusselt's solution for laminar film condensation on a horizontal tube was derived to describe the drainage velocity profile in the liquid film at a cross-section away from the bubble ends and was able to explain the observed tilt of the bubble. Performing a non-dimensional analysis of the model, the ratio of the average drainage to bubble velocities was found to be a function of the Bond and Capillary (Ca) numbers. The gravitational effect on the bubble behavior vanishes for small value of Bo and/or Ca or as Ca -> infinity. Heat transfer experiments in a 2.00 mm tube (Bo = 0.915) showed that gravitational effects had no significant effect on heat transfer in the Taylor flow regime for the conditions studied. (C) 2011 Elsevier Ltd. All rights reserved.