The Legendre-transformed Gibbs energy change for a biochemical reaction, Delta(r)G', is shown to be equal to the nontransformed Gibbs energy change, Delta(r)G, of any single reaction involving selected chemical species of the biochemical system. These two Gibbs energies of reaction have hitherto been thought to have different values. The equality of the quantities means that a substantial part of biochemical and chemical thermodynamics, previously treated separately, can be treated within a unified thermodynamic framework. An important consequence of the equality of Delta(r)G and Delta(r)G' is that the Gibbs energy change of many enzyme reactions can be quantified without specifying which chemical species is the active substrate of the enzyme. Another consequence is that the transformed standard Gibbs energy change of a reaction, Delta(r)G('0), can be calculated by a simple analytical expression, rather than the complex cornputational methods of the past. The equality of the quantities is restricted to Gibbs energy changes and does not apply to enthalpy or entropy changes.