In the phase separation of a binary mixture layer subjected to a vertical temperature gradient spanning the critical temperature, it is known that hexagonal cellular patterns including one droplet in each cell are generated in a limited range of the temperature gradient. In the present study, we investigated experimentally the effect of an initial concentration gradient on such novel phase separation patterns. Here, the initial concentration gradients were varied by changing the diffusion time and dissolution temperature at which the mixture was brought from the two- to one-phase region before the phase separation, and were evaluated using interferometry. As a result, it was found that the diameter of the phase-separated droplet increases as diffusion time and dissolution temperature increase, i.e., as the initial concentration gradient decreases. (c) 2005 Elsevier Ltd. All rights reserved.