New members of the family of complex-perovskite oxides with the formula RCu₃(Mn₃Fe)O₁₂ (R = Ce, Pr, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) have been synthesized and characterized. Polycrystalline samples have been prepared from citrate precursors treated under moderate pressure conditions (2–3.5 GPa) and 1000 °C in the presence of KClO₄ as an oxidizing agent. All the samples have been studied by neutron powder diffraction (NPD) at 300 K and 2 K. These oxides crystallize in the cubic space group Im (no. 204). Mn⁴⁺/Mn³⁺ and Fe³⁺ occupy at random the octahedral B positions of the perovskite structure. These materials have also been characterized by magnetic and magnetotransport measurements. The observed enhancement of T_C along the RCu₃(Mn₃Fe)O₁₂ series is understood as an effect of the chemical pressure on the (Mn,Fe)–O bonds as R³⁺ size decreases. The semiconducting behaviour observed in all of the samples is related with the introduction of Fe at B position. Despite the drastic change of the transport properties, significant negative magnetoresistance values are observed in the Fe-containing compounds both at 10 K and 300 K.