Abstract: Gaseous mixtures are formed by introducing plural gases into a container with a predetermined dilution ratio of one gas in the other, withdrawing the gases from the container and admixing them in a pump, and returning the admixed gases to the container. Further dilutions according to the same or a different dilution ratio can be performed, thereby to obtain a highly dilute gas in accurately predetermined proportions. The gaseous mixtures have utility in inhalation therapy and other processes, and may for example comprise mixtures of oxygen or ammonia in diluents such as nitrogen, carbon dioxide, etc.