A new technique is reported for producing a highly stable, fast-response Hg vapor source for high-quality HgTe and HgCdTe growth. The design principles of the Hg furnace are based on the fact that for choked flow conditions across an orifice the gas flow on the low-pressure side is directly proportional to the pressure on the upstream side of the orifice. Measurements show that this system has a response time of 10 s, a dynamic range of 10−6 to 10−3 Torr, and a long-term stability that exceeds our ability to measure with a conventional ion gauge. Additional measurements show a constant flux output for reservoir temperature changes of 20 °C, a stability of 0.1% over >3 h, and setpoint repeatability of 0.1%. Results on HgTe and homogeneous HgCdTe layer growth are presented.