Sliding bubbles contribute to the boiling heat transfer from an inclined surface. The results of a detailed investigation of steam bubbles sliding under a heated plate have recently been reported by Kenning et al.(1). This gives an opportunity to compare an estimate of the liquid film thickness. made by thermal analysis of the experimental results, with a prediction based on the mechanics of the flow, around the bubble. The fact that the estimates were of similar order (50-100 mum) confirms that, unless evaporation rates from the film are high. the film thickness is determined by flow phenomena rather than by heat transfer. The theory for predicting the film thickness is developed to include the effect of drainage and results are given for a range of slopes and bubble sizes.