A liquid scintillation counter suitable for measuring H3and C14activities is described. A photomultiplier tube with a high sensitivity and signal‐to‐noise ratio is used, and is cooled to −10°C to further reduce the tube noise background. The phototube is mounted in a chassis containing its preamplifier, and the pulses are then led to a high‐stability amplifier scaler. The radioactive materials are dissolved in aliquots of a xylene solution containingp‐terphenyl (1.9 g/liter) and diphenylhexatriene (0.020 g/liter). Substances which quench the scintillations or color the scintillator solution cannot be counted by this method. A minimum of 4.3×10−10curie of H3, or 1.8×10−11curie of C14may be measured by the counter.