A visual study on the phase-change behaviors in a vertical two-dimensional porous structure made of staggered miniature silver-copper circular cylinders has been carried out. Subcooled water was pumped into the porous structure from its bottom due to the capillary action developed in the vicinity of a grooved hearing block placed on the top of the porous structure. Using a high-speed video imaging system, both pore-scale bubble-growth behaviors and continuum-scale distributions of two-phase zone in the porous structure were observed. Photographic results reveal that for a small or moderate heat flux, isolated bubbles formed, grew, and collapsed in the pores in a cyclic manner with a nearly constant frequency. In the macroscopic view, it is found that the periodic downflows of dispersed bubbles and upflows of the liquid phase in the porous structure led to a quasi-steady liquid-vapor two-phase zone. As the imposed heat flux was increased, both the frequency of the bubble growth-collapse cycle and the number of isolated bubbles increased while in the macroscopic view, the two-phase zone expanded laterally but shrank vertically. When the imposed heat flux was sufficiently high, a vapor film was observed beneath the heated fin. These visual observations explain heat transfer measurements: with an increase of the imposed heat flux, the heat transfer coefficient increases to a maximum value and then rapidly decreases afterwards. (C) 2000 Elsevier Science Ltd. Ail rights reserved.