When temperature, pressure, and the partial pressure of a reactant are fixed, the criterion of chemical equilibrium can be expressed in terms of the transformed Gibbs energy G′ that is obtained by using a Legendre transform involving the chemical potential of the reactant that is fixed. For reactions of ideal gases, the most natural variables to use in the fundamental equation are T, P′, and PB , where P′ is the partial pressure of the reactants other than the one that is fixed and PB is the partial pressure of the reactant that is fixed. The fundamental equation for G′ yields the expression for the transformed entropy S′, and a transformed enthalpy can be defined by the additional Legendre transform H′=G′+TS′. This leads to an additional form of the fundamental equation. The calculation of transformed thermodynamic properties and equilibrium compositions is discussed for a simple system and for a general multireaction system. The change, in a reaction, of the binding of the reactant that is at a specified pressure can be calculated using one of the six Maxwell equations of the fundamental equation in G′.