Efficient delivery of oxygen is important during hyperbaric oxygen therapy. We compared two systems in common use, and developed a method to ensure that O2 delivery was adequate during treatment. The systems were a demand valve system with an oral-nasal mask, and a continuously ventilated hood. Five groups were studied over two different time periods, and a further trial was undertaken to examine exhaled O2 levels. The results showed that an acceptable Fio2 could be reliably achieved only with the continuously ventilated hood system or when trained staff supervised their colleagues using the demand system. Inasmuch as the oral-nasal mask system is the standard equipment for the North Sea diving industry, this work shows the importance of ensuring that the correct dose of O2 is delivered. The study indicates, however, that identification of the problem does not always allow a complete solution, and that a hood-based system is more reliable.