The subsolidus phase relations of the SrO–WO3–CuO system were investigated in air. The samples were equilibrated at 800 °C. Under these conditions, eight binary oxides are stable. The pseudo-ternary section contains two ternary oxide phases: the previously described Sr2CuWO6 phase as well as a new phase with Sr8CuW3O18 stoichiometry. The crystal structure of this new compound was solved and refined in the R-3 space group, unit cell parameters a = 5.7202 Å and c = 28.873 Å. Sr8CuW3O18 has a distorted perovskite type structure, where some of the Sr atoms are positioned at the B sites, and the composition may be given as: Sr6(Sr2CuW3)O18. There are indications of existence of a superstructure or stacking faults related to Cu/Sr ordering.