The relative contributions of interstitials and vacancies to diffusion of a dopantAin silicon are specified by the interstitial fraction of diffusivity,fA.Accurate knowledge offAis required for predictive simulations of Si processing during which the point defect population is perturbed, such as transient enhanced diffusion. While experimental determination offAis traditionally based on an underdetermined system of equations, we show here that it is actually possible to derive expressions that give meaningful bounds onfAwithout any further assumptions but that of local equilibrium. By employing a pair of dopants under the same point-defect perturbance, and by utilizing perturbances very far from equilibrium, we obtain experimentallyfSb⩽0.012andfB⩾0.98at temperatures of∼800 °C,which are the strictest bounds reported to date. Our results are in agreement with a theoretical expectation that a substitutional dopant in Si should either be a pure vacancy, or a pure interstitial(cy)diffuser. ©1997 American Institute of Physics.