The ratio \[\frac{(x_{1}/x_{2})_{\rm{A}}}{(x_{1}/x_{2})_{\rm{B}}}\] where \(x\) is the abundance, expressed as the atom fraction of the isotope distinguished by the subscript numeral, when the two isotopes are equilibrated between two different chemical species $\ce{A}$ and $\ce{B}$ (or between specific sites A and B in the same or different chemical species). The term is most commonly met in connection with deuterium solvent isotope effects, when the fractionation factor \(\mathit{\Phi}\) expresses the ratio: \[\varPhi = \frac{(x_{\rm{D}}/x_{\rm{H}})_{\rm{solute}}}{(x_{\rm{D}}/x_{\rm{H}})_{\rm{solvent}}} \] for the exchangeable hydrogen atoms in the chemical species (or sites) concerned. The concept is also applicable to transition states.