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Two cationic heterometal–organic frameworks (Eu-Zn (1·NO3−) and Tb-Zn (2·NO3−)) with NO3− counter-anions in the channels are structurally and luminously characterized. Both of them can serve as highly sensitive and highly selective luminescent probes for detecting I− ions in aqueous solutions. In particular, 2·NO3− can selectively and reversibly detect I− with a fast response time of just 10 s and an extremely low detection limit of 0.001 ppm. Mechanism studies reveal that I− is quickly oxidized to form I3− with the help of 1·NO3− or 2·NO3−, leading to luminescence quenching. This represents the first report of MOF-based luminescent probes for the detection of I− in aqueous solutions. [ABSTRACT FROM AUTHOR]