The deuterium content of mixtures of water and heavy water can be determined by means of a nondispersive infrared gas analyzer. An apparatus is described to measure D/sub 2/O content in vapor mixtures from the natural concentration range to 40% D/sub 2/O with an accuracy of about 0.5% D/sub 2/0. Much better results can be achieved with liquid mixtures. The method is suitable for rapid determinations on flowing streams of water. (W.D.M.)