It is well-known that the process of mixing two solutions of different concentrations may produce energy. The change in Gibbs energy upon mixing is believed to be the source of energy. However, the change in the Gibbs energy is independent of the way of mixing, i.e., irreversible (without work production) or reversible (with the obligatory production of work). The Gibbs energy cannot be an energy Source for producing work. The entropy of mixing solutions and the environment has been considered to elucidate the role of the Gibbs energy in the reversible process. It is shown that the environment takes active part in energy production. The Gibbs energy may be a mathematical means for performing thermodynamic calculations. Published by Elsevier B.V.