EN
This research was undertaken to determine whether by powder diffraction substitution in the "123"-type high T_{c} oxides, Ln_{1-x}M_{x}Ba_{2-y}M_{y}Cu_{3-z}M_{z}O_{6.5+δ} (where Ln = lanthanoid, M_{x} = Me^{3+}, M_{y} = Me^{3+} and M_{z} = Me^{3+} and Me^{2+}) could be detected unequivocally. Numerous X-ray and a few neutron powder diffractograms were calculated for substituted compounds. Reflections most sensitive to substitution and the influence of site occupancy for Cu and O atoms are also characterized. The results are compared with some experimental data reported in the literature.