The design and implementation of a pressure cell for the study of hydrothermal reactions byinsituenergy dispersive x‐ray diffraction is described. The cell permits the study of both the kinetics and mechanism of formation of a wide range of important solid‐state compounds such as zeolites and other microporous solids. Reactions can be studied over a wide range of temperature (5–230 °C) and autogenous pressure [0–400 psi (gauge)] conditions. The use of this apparatus is illustrated by a study on the synthesis of a microporous tin chalcogenide phase performed on Station 9.7 of the UK Synchrotron Radiation Source. ©1995 American Institute of Physics.