Electrochemical impedance and new measurement techniques based on the sinusoidal response of non-electrical quantities are discussed here based on the theory of non-equilibrium thermodynamics. The theory of non-equilibrium thermodynamics or thermodynamics of the irreversible processes provides us with a general framework for the macroscopic description of irreversible processes, such as electrochemical systems. According to this theory, the equation for the entropy balance plays a major role. Several different irreversible processes can be separated and classified using this equation, as well as the experimental limitations which arise from the second principle of thermodynamics. This paper shows, from the thermodynamics standpoint, the relationship linking a perturbation with a given response. It also shows, the conditions under which it becomes apparent that there is a "reciprocity" between "conjugate properties", as is the case with current/potential (i.e. potentiostatic or galvanostatic control) and that this is nothing but a consequence of an experimental artifice. (C) 2004 Elsevier Ltd. All rights reserved.