Abstract: An apparatus for measuring the in situ levels of tritium in ground water at depth in the earth. A tritium analyzer is made to fit in a sonde or probe which is placed in a borehole. This analyzer can perform a programmed cycle and has a sample intake to allow ambient water to enter; a reaction chamber; a drying chamber; an ion chamber; a cryogenic gas pump, and a spent capsule collection chamber. After the water sample is brought into the unit, it rises into the reaction chamber where it reacts with a preweighed quantity of calcium carbide in a capsule to yield acetylene. Next the acetylene vapor passes through the drying chamber to remove excess water and then flows into the evacuated ion chamber. Following this, the ion chamber is sealed off and a count of tritium beta decay events is started. Following the completion of the count, a valve is opened to remove the acetylene from the ion chamber with the cryogenic gas pump.