In situ X-ray studies have been widely used in both solid-state chemistry and wet-chemistry to study a variety of reactions and processes, but so far studies of syntheses in the supercritical phase are few and mainly limited to static experiments. Here the design of reactor cells for time-resolved in situ X-ray studies of synthesis in supercritical water is discussed and examples of the information that can be obtained are given for both batch and continuous flow reactors. It is shown that highly useful information on process kinetics can be obtained despite the considerable experimental challenges. (C) 2007 Elsevier B.V. All rights reserved.