[en] Calculations with two-group perturbation theory on substitution experiments with homogenized regions show that a condensation of the results into a one-group formula is possible, provided that a transition region is introduced in a proper way. In heterogeneous cores the transition region comes in as a consequence of a new cell concept. By making use of progressive substitutions the properties of the transition region can be regarded as fitting parameters in the evaluation procedure. The thickness of the region is approximately equal to the sum of 1/(1/τ + 1/L²)^1/2 for the test and reference regions. Consequently a region where L² >> τ, e.g. D₂O, contributes with √τ to the thickness. In cores where τ >> L² , e.g. H₂O assemblies, the thickness of the transition region is determined by L. Experiments on rod lattices in D₂O and on test regions of D₂O alone (where B² = - 1/L² ) are analysed. The lattice measurements, where the pitches differed by a factor of √2, gave excellent results, whereas the determination of the diffusion length in D₂O by this method was not quite successful. Even regions containing only one test element can be used in a meaningful way in the analysis