The term "mechanochemical system" is used to designate thermodynamic systems capable of "transforming" chemical energy directly into mechanical work or conversely of "transforming" mechanical into chemical energy. These transformations can be carried out in an "engine" capable of performing cyclically, reverting after each cycle to its initial state. The theoretical analysis of the mechanical systems presented in this paper is not concerned with the particular behavior of either natural or synthetic contractile fibers. It is devoted to the formulation of the necessary conditions for mechanochemical performance and to the adaptation of classical thermodynamic concepts and equations to the study of mechanochemical systems. Some applications to living systems are also considered.