A digital photographic study of subcooled flow boiling with R-134a was performed in a vertical rectangular channel heated from one side to observe near-wall structures. Primary attention was given to bubble formation and bubble coalescence in the bubble layer. The visualization made possible a detailed observation of the characteristic near-wall region, consisting of vapor remnants, an interleaved liquid layer, and coalesced adjacent bubbles. In particular, the visualization provided clearer insights about bubble coalescence behaviors during flow nucleate boiling. In addition, it is shown that the near-wall bubble layer of nucleate boiling beneath vapor clots is extinguished and, afterwards, the heated surface is locally covered by large vapor films, at CHF. (C) 2004 Elsevier Ltd. All rights reserved.