Cavity formation is an important phenomenon in the underground coal gasification (UCG) process. In the early stages of cavity formation, only the combustion reaction is performed in order to stabilize the temperature field. In the present work, we study the formation of the combustion cavity and the effect of various design and operating parameters such as the distance between the wells, feed flow rate and operation time, on its evolution. This paper presents details of laboratory-scale experiments that demonstrate the shape and size of the combustion cavity, and their dependence on various parameters. Empirical correlations for the cavity volume and dimensions in various directions are developed, which indicate the strong effects of mass transport. Results from computational fluid dynamics (CFD), which map the velocity profiles in the cavity, support the experimental observations. (C) 2010 Elsevier Ltd. All rights reserved.