Abstract <p>High-level waste is generated during the reprocessing of spent nuclear fuel. The strategy for managing such waste involves its fractionation, primarily aimed at extraction and separation of americium and curium. The separated fraction containing curium and rare earth elements (the Cm/REE fraction) is subject to temporary storage, which highlights the need for the development of durable matrices for its immobilization. In this work, samples of a magnesium potassium phosphate compound containing up to 10.7 wt % of a REEs mixture (including Nd, La, Ce, Sm, Gd as a simulant of the separated Cm/REE fraction) were synthesized. The phase composition of the compound, its compressive strength, resistance to freeze–thaw thermal cycling, water resistance, and hydrolytic stability were determined. The compound exhibits high hydrolytic stability: the leaching rate of REEs after 365 days of contact with water is approximately 7.4 × 10^–7 g/(cm² day). It was established that the quality indicators of the compound containing up to 6.4 wt % REEs meet the regulatory requirements for solidified radioactive waste.</p>