Caesium phosphomolybdate (Cs3PMo12O40·xH2O) and zirconium molybdate ([ZrMo2O7(OH)2]·2H2O) solids are known to precipitate out from highly active liquors (HAL) during reprocessing of spent nuclear fuel. Here, a new synthesis for these simulants is reported; with the initial step producing spherical ceasium phosphomolybdate particles, which can then be converted into cubic Zirconium molybdate. Additionally, the addition of citric acid prior to the formation of the zirconium salt is investigated. In this case, a citratomolybdate complex is generated, leading to the synthesis of elongated cuboidal zirconium citratomolybdate ([ZrMo2O7(OH)2]·2H2O·[(MoO2)2O(cit)2]). A key focus of this study is to explore the optimisation of reaction conditions to create a controlled environment for the particles to form with high conversion rates and with desired shape properties. Elemental and structural characterisation of the particles at various points during the synthesis, as well as post-synthesis, was undertaken to provide further insights. Ultimately, it is of importance to determine the mechanism of how these simulants are formed within the components in HAL. Establishing the influence of particle properties on HAL behaviour is key for current processing, post operational clean out (POCO) and life-time assessment of the nuclear waste facilities.