Single phase materials of RuSr2Gd0.5Eu0.5Cu2O8(Ru‐1212Gd0.5Eu0.5) and Ru(Sr1−xNax)2GdCu2O8(0 ⩽ x ⩽ 0.10) have been successfully synthesized in oxygen at 1065 °C. DC‐resistivity and magnetic susceptibility measurements show a decreases of the superconducting transition temperatureTcfrom 46 K in the pure Gd compound to 41 K in Ru‐1212Gd0.5Eu0.5, whereas the magnetic ordering temperatureTMdoes not change. The influence of the monovalent substitution Na+for Sr2+on the properties of Ru‐1212Gd is investigated and compared with previous work. Finally, the effects of Cu substituted RuSr2Gd(Cu1−xMx)2O8compounds, where M is Co, Ni and Ga for 0 ⩽x⩽ 0.03 are reported and compared with similar doping experiments in other high‐Tcsuperconductors. © 2003 American Institute of Physics