The behavior of near-wall bubbles in subcooled flow boiling has been investigated photographically for water flow in vertical, one-side heated, rectangular channels at mass fluxes of 500, 1500, 2000 kg.m(-2).s(-1) under atmospheric pressure. Primary attention was given to the bubble coalescence phenomenon and the structure of the near-wall bubble layer. The number of near-wall bubbles increased with the increase in the heat flux. At sufficiently high heat fluxes (>60-70% CHF), three characteristic layers were observed in the heated channel: (a) a superheated liquid layer with small bubbles attached on the heated wall, (b) a flowing bubble layer consisting of large coalesced bubbles over the superheated liquid layer, and (c) the liquid core over the flowing bubble layer. In addition, the existence of a liquid sublayer under coalesced bubbles was identified photographically. According to visualization, the CHF mechanism for the present experimental condition could be related to the formation of large vapor clots resulting from coalescences of bubbles and the evaporation of the superheated liquid layer beneath those clots. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.