Abstract A rigorous analysis of the thermal processes involved in the change of state of a saturated fluid shows that four types of calorimetric processes each yield values of a characteristic thermal property of the fluid. This group of calorimetric properties may be used to formulate the thermal behavior of the fluid in terms of familiar and convenient properties within the range of the experimental possibilities. The principles of a calorimetric method of determining the thermodynamic characteristics of a fluid are outlined. The method has been developed at the Bureau of Standards and used in the determinations of the properties of saturated water and steam, published simultaneously with this paper. This paper supplements that report and furnishes the theoretical groundwork for the experimental work. The results of the analysis have been used as a guide in developing a systematic experimental project which has resulted in the determination of an important group of properties of saturated steam.