NbM 2RCu 2O 8 and TaM 2RCu 2O 8 (M Ba, Sr; R Pr, Nd, Sm, Eu and Gd) phases were synthesized. Electron diffraction study of TaSr 2EuCu 2O 8 showed the presence of a weak superstructure ( a = 82a 07), like that found for a number of similar phases. Rietveld refinement of this structure based on X-ray powder diffraction data was carried out in the space group P4/ mmm ( R 1 = 0.038, R p = 0.037) since no superstructure reflections were observed. Various types of cation replacement were performed in order to increase the copper valence. Monophasic Nb(Ta) 1, Ti,Sr 2EuCu 2O 8 samples were synthesized at P( O 2) = 1 bar and T = 1050°C up to x = 0.1 (Nb) and 0.3 (Ta), but no superconductivity was detected down to 12 K. High pressure syntheses were carried out to obtain Nb(Ta)Sr 2Eu 1, Ca, Cu 2O 8 phases, but they were found to have a narrow homogeneity range that provides no opportunity of inducing superconductivity in such a way.