The new complex perovskites of formula R(Cu3-xMnx)Mn4O12 (R=Pr, Nd and x=1,2) have been prepared. The partial replacement of Cu2+ by Mn3+ cations does not involve a change in symmetry from that of the parent compounds RCu3Mn4O12, presenting a cubic structure with space group Im (3) over bar. The Mn3+ introduced at the square-planar 6b site promotes the valence mixing of Mn3+ and Mn4+ ions at the octahedral 8c site, leading to an increase in the bonding distances in the MnO6 octahedron and a slight decrease in the distances in the CuO4 polyhedron. All the compounds present a spontaneous increase in the magnetization below T-C, typical of ferro-or ferrimagnetic materials. Due to the increasing content of Mn3+ at the 8c sites, some Mn-Mn exchange interactions switch from ferromagnetic (FM) to antiferromagnetic (AFM), weakening the overall FM exchange interactions and decreasing T-C and M-S. Neutron powder diffraction (NPD) data indicate that both sublattices (square-planar and octahedral) show a parallel arrangement of the magnetic moments. (C) 2010 American Institute of Physics. [doi:10.1063/1.3498807]