The high-dilution diffusion of Tl+ in molten NaNO3 , KNO3 and RbNO3 has been measured with wave-front-shearing interferometry. For very low concentrations of TlNO3 the results are D = 6.21×10-8 exp [-(18400± 800)/RT] in NaNO3 (316-387 °C), D = 5.39×10-8 exp [-(18800±1400)/RT] in KNO3 (341-408 °C), D = 1.87×10-7 exp [-(25800±2200)JRT] in RbNO3 (327-419°C), where D is the diffusion coefficient in m2 s-1, R the gas constant in J K-1 mol-1 and T the temperature in K. Comparisons are made with high-dilution diffusion of Na+, K+, Rb+, Cs+ and Ag+ ions in alkali nitrates, and it is found that both Ag+ and Tl+ diffuse faster than expected if the mass and the ionic radius were the sole parameters of importance, i. e. other parameters such as the polarizability also have an influence on the diffusion rate of an ion. For the system NaNO3 - TlNO3 , the interdiffusion coefficient has also been measured at 88.8, 73.8, 51.5 and 31.6 mol % NaNO3 .