Titanate-based hollandite ceramics are promising nuclear waste forms for Cs immobilization. In this work, a series of Al-substituted hollandite (Ba,Cs)(1.33)(Al,Ti)(8)O(16)was investigated across a broad compositional range with varying Cs content. Powder X-ray diffraction showed that all samples exhibited a tetragonal hollandite phase. Enthalpies of formation determined by high-temperature melt solution calorimetry indicated enhanced thermodynamic stability with increased Cs content, which generally agreed with sublattice-based thermodynamic calculations. Moreover, enthalpies of formation of the samples were primarily affected by three factors: (a) relative sizes of cations on the A-sites and B-sites, (b) tolerance factor, and (c) optical basicity. Fractional element release revealed that Cs retention was significantly improved for the high Cs-containing hollandite compositions, which were supported by the evolution of microstructure of the pre and postleach particles. Elution studies of Al-substituted hollandite spiked with radioactive(137)Cs indicated that transmutation of Cs to Ba in the hollandite was accompanied by an increase in the retention of the Cs decay product, suggesting long-term stability of Al-substituted hollandite phase.