This study investigated the release mechanism of iodine in Pb9.85(VO4)6I1.75 to evaluate its long-term chemical durability. Leaching tests were conducted in different solutions: deionized water, pH buffers, and 0.1 mol/L of NaCl, Na2CO3, Na3PO4, and Na2SO4. Typically, a sample was leached in leachant, replaced every 24-hour, under 90±0.5°C using a semi-dynamic protocol. Solution and surface characterization show the iodine release is subjected to both diffusion and dissolution processes and is highly sensitive to the solution chemistry: 1) in pH neutral solutions, iodine release is controlled by short-term diffusion via ion-exchange and long-term dissolution; 2) in acidic and basic solutions, dissolution dominates the iodine release; 3) in ion-rich solutions, iodine release is enhanced by rapid ion-exchange and increased ionic strength. In addition, we also observed secondary phases including: Pb2V2O7 from pH 4, Pb5(VO4)3Cl from NaCl, and Pb5(VO4)3OH from pH 10. This study provides implications for long-term disposal safety of nuclear waste.