New cation-deficient perovskite-like oxides Ba5MTa4ZrO18and Ba5MTa4Zr2O21(M=Ba, Sr) were synthesized and studied by X-ray diffraction, electron diffraction, and high-resolution electron microscopy (HREM). The Ba5SrTa4ZrO18(space groupR3m, (hcccch),a=5.80162(8) Å,c=42.5403(8) Å,RI=0.039,Rp=0.082) and Ba5Sr2Ta4ZrO21(space groupR3m, (hccccch)3,a=5.7998(1) Å,c=49.595(1) Å,RI=0.033,Rp=0.080) crystal structures were refined from X-ray powder diffraction data. The results of structure refinements were confirmed by HREM observations. The formation of the Ba5Sr3Ta4Zr3O24compound was revealed and the structure of this compound was deduced from HREM (space groupP3m1,hcccccch,a≈5.8 Å,c≈18.9 Å). All these compounds belong to theAnBn-1O3nhomologous series of cation-deficient hexagonal perovskite-like oxides withn=6, 7, and 8 and may be formally described as intergrowth structures with alternating Ba5Ta4O15-type and Ba(Sr)ZrO3-type structural blocks along thecaxis.