A high-vacuum specimen chamber was constructed for a Siemens electron microscope in order that in situ studies of the growth of refractory metal films may be performed under clean well-defined conditions. The chamber is differentially pumped and reaches a vacuum of 0.1 mu Torr with the rest of the column at about 20 mu Torr. The hydrocarbon contamination rate at the specimen is approximately 2.5 AA min^-1 compared with about 500 AA min^-1 for the unmodified instrument. The stage operates smoothly, and images at 80000* are steady and free from vibration.