Of component i, defined by the equation \[\mathit{\Gamma}_{i}^{(n)} = \mathit{\Gamma}_{i}^{\unicode[Times]{x3C3} }- \mathit{\Gamma}_{i}\ \frac{c_{i}^{\unicode[Times]{x3B1} } - c_{i}^{\unicode[Times]{x3B2} }}{c^{\unicode[Times]{x3B1} }- c^{\unicode[Times]{x3B2} }}\] where Γ σ, cα and cβ are, respectively, the total Gibbs surface concentration and the total concentrations in the bulk phases α and β: \[\mathit{\Gamma}^{\unicode[Times]{x3C3} }=\sum _{\begin{array}{c} i \end{array}}\mathit{\Gamma }_{i}^{\unicode[Times]{x3C3} }\] \[c^{\unicode[Times]{x3B1} }=\sum _{\begin{array}{c} i \end{array}}c_{i}^{\unicode[Times]{x3B1} }\] \[c^{\unicode[Times]{x3B2} }=\sum _{\begin{array}{c} i \end{array}}c_{i}^{\unicode[Times]{x3B2}}\] The reduced adsorption is invariant to the location of the Gibbs surface. Alternatively, the reduced adsorption may be regarded as the Gibbs surface concentration of i when the Gibbs surface is chosen so that Γ σ is zero, i.e. the Gibbs surface is chosen so that the reference system has not only the same volume, but also contains the same total amount of substance(n) as the real system.