[en] The material buckling of D₂O-enriched uranium (2% U₂₃₅) system with voided fuel assembly has been determined by means of the substitution technique. The experiments were carried out in the zero power reactor RB for various lattice pitches, namely: 11.3 cm, 14 cm, and 16 cm. As a reference core the D₂O-enriched uranium (2%U₂₃₅) ) system without voids has been used. The results have been analysed by the modified one-group perturbation theory using three regions in a substituted reactor (test, intermediate and reference). The perturbations in axial and radial directions due to voided fuel assembly, are also considered. In order to determine the change between diffusion coefficients in various regions (δD), additional experiments with single test fuel element were made. The lattice pitches of reference and test regions were the same. According to the algorithms given in appendices, the programmes for the ZUSE Z-23 digital computer were made. These programmes include evaluations of the change between diffusion coefficients (δD), and the axial buckling of the test lattice (author)