A new method for determining the donor-acceptor distribution function in fluorescence resonance energy transfer (FRET) systems is presented. The approach is based on time-resolved fluorescence experiments with nanosecond resolution. Potential applications of this method include: determination of the morphology of porous materials (e.g., polymers, sol-gels, resins, etc.), monitoring processes occurring on the nanometer scale including biomolecules labeled with the donor/acceptor species, and FRET sensors based on competitive binding. In this paper a theoretical derivation of the method is presented and the method is tested in a series of numerical simulations. The experimental conditions regarding this approach are discussed and its applicability to real measurement systems is demonstrated.