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 $\mathit{\Phi }$ expresses the ratio: $$\Phi =\frac{(x_{\text{D}}/x_{\text{H}}{)}_{\text{solute}}}{(x_{\text{D}}/x_{\text{H}}{)}_{\text{solvent}}}$$ for the exchangeable hydrogen atoms in the chemical species (or sites) concerned. The concept is also applicable to transition states.