Performance of entrained-flow coal gasifiers is in large part dictated by the burnout behavior of coal particles. In particular, the transition from porous, reactive char to molten, low reactive slag affects overall coal conversion. In this work, the physical phenomena associated with char-slag transition were studied for three coals using a laminar entrained-flow reactor under simulated gasification conditions. Partially oxidized particles with various conversions were prepared at temperatures above the ash fluid temperatures. The physical properties of the char and slag particles were characterized, including the particle density, size, internal surface area and morphology. Results show that at a coal-dependent critical conversion, the particles undergo remarkable physical changes, such as density increase, size reduction and surface area decrease. These phenomena indicate the char-slag transition. (C) 2011 Elsevier B.V. All rights reserved.