The ratio \[\frac{(x_{1}/x_{2})_{\text{A}}}{(x_{1}/x_{2})_{\text{B}}}\] 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 A and 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 Φ expresses the ratio: \[\varPhi = \frac{(x_{\text{D}}/x_{\text{H}})_{\text{solute}}}{(x_{\text{D}}/x_{\text{H}})_{\text{solvent}}} \] for the exchangeable hydrogen atoms in the chemical species (or sites) concerned. The concept is also applicable to transition states.