For many years the author had devoted increased attention and study to the problems of the change of state and quantity of thermal working substances. These thermal processes particularly occur in piston engines, e.g., in steam engines during admission and release, in internal-combustion engines in the exhaust, and in compressors during the inlet to and the outlet from the cylinder. In turboengines they also can play a part, e.g., in gas turbines with explosion chambers, such as the Holtzwarth turbine. In the author’s former essay numerous instances based on the first law of thermodynamics have been dealt with whereas, in this paper, first some other applications of the first law of thermodynamics and then the formulation of the second law of thermodynamics for changes of state and quantity will be presented. Particular attention will be devoted to the thermodynamic aspect of the entropy diagrams dealing with variable quantities, and the sources of errors which may occur when using such diagrams.