An experimental study has been carried out in order to investigate the two-phase flow structure in the vicinity of a vertical heated wall (120 mm high) on which boiling of pentane takes place. Measurements of the frequency and time of bubble passages and also of void fraction have been performed by means of hot-wire anemometry (HWA). It has been shown that, at high heat flux, bubbles mainly remain close to the wall and form large coalesced bubbles. On the contrary, at low or moderate heat flux, they are propelled into the fluid core prior to rising in a bubble column located about 1.5-2 nun away from the heated wall and sometimes they coalesce during their rising motion in the upper part of the column. Using a classification of bubbles based on their passage time, the contribution from short, average and long bubbles to the void fraction was also discussed. This classification allows to determine the transition between the boiling two-phase flow patterns defined previously from objective criteria: the regime of isolated bubbles or partial nucleate boiling occurs for q'/q'(crit) < 0.2, the fully developed nucleate boiling for 0.2 < q'/q'(crit) < 0.7 and the transition to critical heat flux (CHF) for q'/q'(crit) > 0.7.