The blue emitting luminescent MOF ³_∞[Ce(Im)₃ImH]·ImH forms a 3D-framework with Kagomé net topology. The framework exhibits an intense blue luminescence which can be retained upon activation of the MOF with the formula ³_∞[Ce(Im)₃ImH]. The luminescence is metal-based due to parity-allowed 5d–4f-transitions. Time-dependent investigations of the interaction with liquid and gas analytes show that the MOF – by utilising 5d–4f-transitions of Ce³⁺ – can be used as a high-speed “turn-off” detector for water and oxygen in dry air. Other protic or polar solvents, like methanol, acetone or pyridine, which also show a “turn-off”-effect can be distinguished from water-detection either on a time scale (ranging up to 250 000 : 1) or a shift of the chromaticity, the latter being pronounced for MeOH. The fast time-dependent decrease of the luminescence intensity for water arises from an extremely fast hydrolysis and is irreversible. Polar aprotic molecules like dichloromethane and acetonitrile can also result in a “turn-on”-effect of the luminescence intensity due to their behaviour as additional sensitizers for Ce³⁺-emission. We conclude that the cerium-MOF can be utilised in gas and liquid sensing applications as a detector material for water and oxygen in dry air. The luminescence is intense with good quantum yield between 55% (as-synthesised) and 36% (activated). This implies that only milligram amounts of the material are needed to detect the analyte species and is especially useful, as the MOF can be directly used as-synthesised for water detection in applications for which an irreversible signal change is desired, e.g. preventing a signal change upon unwanted re-drying.