The concept of saturation diving has, for the first time, provided promise that practical undersea work can be carried out at depths approaching 1000 feet. The principal objective of this manual is to provide the best available information on gas properties in a form convenient for use in diving research, engineering, and operations. All of the data in this manual are based upon calculation from theoretical relationships, substantiated where experimental information can be found in the literature (as for pure gases), and unsubstantiated where such information does not exist (as for helium-oxygen mixtures). It is felt that the data presented are the best that can be generated today, and that they will be generally satisfactory in the pressure range up to about 500 psi. For the higher pressures used in storing of diving gases, errors are indeterminate and may be significant. Future experimental research is needed to improve the state of knowledge of mixture properties at very high pressures, and to explore the properties of helium-oxygen-nitrogen mixtures. A second objective of this manual is to summarize the present practice regarding choice of breathing-gas mixtures and some of the calculation procedures used in design and operation of diving equipment. An attempt has been made to present these procedures in such a manner that they can be used by both designers and operating personnel.