A method for determining deuterium oxide content in the concentration range of 1 to 100% with a precision of 1 to 0.1 percentage unit is described. Measurements were made with a glass cell 0.6 mm thick using the infrared spectrum region from 4000 to 4400 cm−1. It was found that glass windows can be also used for absorption cells in the range between 2300 and 3800 cm−1in which very intense OD and OH bands appear.