The compositions of the general formula Ln(11-x)Sr(x)Ir(4)O(24) (Ln = La, Pr, Nd, Sm; 1.37 >= x >= 2) belonging to a family of A-site cation-deficient double-perovskite-related oxide iridates were grown as highly faceted single crystals from a molten strontium chloride flux. Their structures were determined by single-crystal X-ray diffraction. On the basis of the single crystal results, additional compositions, Ln(9)Sr(2)Ir(4)O(24) (Ln = La, Pr, Nd, Sm), were prepared as polycrystalline powders via solid-state reactions and structurally characterized by Rietveld refinement. The compositions Ln(9)Sr(2)Ir(4)O(24) (Ln = La, Pr, Nd, Sm) contain Ir(V) and Ir(IV) in a 1:3 ratio with an average iridium oxidation state of 4.25. The single-crystal compositions La9.15Sr1.85Ir4O24 and Pr9.63Sr1.37Ir4O24 contain relatively less Ir(V), with the average iridium oxidation states being 4.21 and 4.09, respectively. The magnetic properties of Ln(9)Sr(2)Ir(4)O(24) (Ln = La, Pr, Nd, Sm) were measured, and complex magnetic behavior was observed in all cases at temperatures below 30 K.