<p>Actinides are the major components of spent nuclear fuel and thus critical for the long-term safety assessment of deep geological repository (DGR). The mobility of actinides is largely dependent on aqueous species, solubility, and interactions with mineral surfaces under repository-relevant conditions. This review highlights environmental factors that can govern the migration and retardation of actinides in the chemical thermodynamic context. The oxidation states of actinides, and thus the balance between soluble and sparingly soluble species, are primarily controlled by pH and redox potential. Among natural inorganic ligands, carbonate strongly stabilizes hexavalent actinides, particularly uranium, while ternary complexes with alkaline earth metals (e.g., Ca-UO₂-CO₃) further enhance the stability in aqueous solution and limit sorption affinity. Ionic strength influences activity coefficients, adsorption processes, and colloid stability, whereas elevated temperature modifies equilibrium constants for hydrolysis and complexation reactions. Collectively, these parameters dictate the extent to which actinides remain in solution as mobile species or are immobilized by precipitation and sorption. A consistent thermodynamic framework is therefore required to describe their behavior across various geochemical environments. Ongoing efforts to refine thermodynamic databases and validate key reactions remain essential for a reliable and robust safety assessment of a high-level radioactive waste repository.</p>