A system for performing x‐ray diffraction on biological samples as a function of pressure and temperature is described. It is capable of operating in a pressure range of 1 bar–3 kbar (0.1–300 MPa) and in a temperature range of −30 to 80 °C. The system incorporates microprocessor‐based pressure and temperature controllers which provide automated control with excellent stability characteristics: Fluctuations in pressure and temperature can be maintained within ±1 bar (0.1 MPa) and ±0.05 °C, respectively. Use of the apparatus is illustrated by application to a pressure‐induced phase transition in a lipid‐water liquid crystal.