A previous study revealed Cs_2TiNb_6O_1_8 to be the major Cs-containing phase after hot isostatic pressing Cs-loaded IONSIV (a commercial exchanger) which demonstrated excellent wasteform properties. Both experimental and theoretical studies have been carried out in order to assess if Cs_2TiNb_6O_1_8 is able to retain ^1^3^7Ba^2^+, the transmutation product of ^1^3^7Cs^+. A series of samples with different charge compensation mechanisms have been synthesised including Cs_2_-_xBa_xTi^(^4^+^)_1_+_xNb^(^5^+^)_6_-_xO_1_8, Cs_2_-_xBa_xTi^(^3^+^)_xTi^(^4^+^)_1_-_xNb_6O_1_8 and Cs_2_-_xBa_xTiNb^(^4^+^)_xNb^(^5^+^)_6_-_xO_1_8. Analysis suggested that Ba incorporation was not successful because of the identification of Ba impurities in the X-ray diffraction patterns. A series of atomistic simulations have also been performed to support the experimental work, using the General Utility Lattice Program code which suggested that Ba incorporation is not energetically favourable. Sr-loaded IOSNIV has also been thermally converted via calcination (in air) and hot isostatic pressing. The removal and immobilisation of Sr is an important process on account of ^9^0Sr being one of the more problematic radionuclides produced from the fission process. Both thermal conversion methods produced crystalline phase assembles which were analysed by X-ray diffraction, X-ray fluorescence and microscopy studies. The HIPed materials performed well in aqueous durability tests, suggesting these wasteforms will be suitable for final disposal in a geological disposal facility.