The severe accident at the Fukushima-Daiichi Nuclear Power Station in 2011 has shown the necessity to study the impact of the release of hazardous fission products. This work investigates the Ba-Cs-Sr-Mo-O system, which contains some of the most abundantly produced fission products, as well as fission products that carry a great health risk on release. The study of this system is broken up into four subsystems: Ba-Sr-O, Ba-Mo-O, Sr-Mo-O and Ba-Cs-Mo-O. A literature study into the ternary Ba-Sr-O system, including existing thermodynamic models, showed the formation of no stoichiometric ternary compounds due to the mutual miscibility of Ba and Sr. Despite this mutual solubility, a miscibility gap is shown to be present in the solid region of the binary BaO-SrO phase diagram below a certain temperature. Thermogravimetric differential scanning calorimetry (TGDSC) investigations of the BaMoO₄ – MoO₃, SrMoO₄ – MoO₃ and BaMoO₄ – Cs₂MoO₄ pseudo-binary systems revealed likely compositions for the eutectic equilibria at 0.792 ≤ x(MoO₃) ≤ 0.80, 0.806 ≤ x(MoO₃) ≤ 0.82 and 0.909 ≤ x(Cs₂MoO₄) ≤ 0.976, respectively. These measurements also allowed for the development and optimisation of a new thermodynamic model of the BaMoO₄ – Cs₂MoO₄ system using the CALPHAD (Calculation of Phase Diagram) method. Syntheses of BaMoO₄, BaMo₃O₁₀, Ba₂MoO₅ and BaCs₂(MoO₄)₂ were successfully completed. A partially successful synthesis method was developed for Ba₃MoO₆ that needs further optimisation. The novel synthesis of Ba₂MoO₅ allowed for solution calorimetry measurements to be performed, leading to the determination of its standard enthalpy of formation Δ_fH°_m(298.15K, Ba₂MoO₅) = -(2169.0 ± 14.7) kJ/mol. Vapour pressure studies of BaMoO₄ by means of Knudsen EffusionMass Spectrometry (KEMS) gave insight into the composition of the vapour formed above BaMoO₄ after vaporisation. The results showed extensive influence of fragmentation reactions and only a small amount of congruent evaporation, indicated by the high partial pressure of BaO(g) and other binary molecules. A partial reduction of the BaMoO₄ sample to BaMoO₃ could have occurred, but this cannot be confirmed due to the full evaporation of the KEMS sample. Further studies are required to investigate a potential reduction.