The phase separation behavior in a layer of an isobutyric acid (IBA)-water binary mixture was observed from the bottom and side of the layer, where the layer was subjected to a vertical temperature gradient spanning the critical temperature. As a result, the following conclusions were obtained. (1) Patterns characterized by a polygonal cell including one droplet, similar to those observed by Assenheimer et al. (Physica A 208 (1994) 373), were formed in a relatively thick liquid layer, e.g., 3 mm thick, in a limited range of the temperature gradient, although the patterns ultimately disappeared. (2) The growth and extinction rates of the phase-separated droplets increased with DeltaT/d, and increased particularly rapidly at high DeltaT/d. (3) The formation of the pattern of polygonal cells each with a droplet is considered to be due to thermocapillary motion along the droplet. (4) The concentration-driven buoyancy convection affects the growth rate of the phase-separated droplets. (5) The extinction rate of the phase-separated droplets at the late stage is affected by the thermocapillary motion along the droplet surface. (C) 2004 Elsevier Ltd. All rights reserved.