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• The ferrocene-based probe colourimetricaly/electrochemicaly sensed HSO 3 − in a 95 % aqueous solution. • Mechanism of sensing was illustrated by 1H & 13C NMR, HR-MS and DFT/TD-DFT analysis. • The plausible mechanism of sensing is via nucleophilic addition of HSO 3 − ion to olefinic C-atom. • The proposed sensing mechanism was well supported by DFT/TD-DFT calculations. • The probe was utilized to estimate the levels of HSO 3 − ions in natural water and red wine samples. A simple ferrocene-based organic probe (AK5) is described for selective and sensitive colourimetric/electrochemical detection of HSO 3 - ions in a 95 % aqueous solution. Upon adding HSO 3 − ions, the colour of the solution changes from brown to colourless instantaneously. The plausible mechanism of detection of HSO 3 − ions by AK5 was illustrated by 1H NMR, 13C NMR, HR-MS and DFT/TD-DFT analysis, which revealed nucleophilic addition of HSO 3 − ion to olefinic C-atom. The limits of detection (LOD) are determined to be 5.58 and 0.12 µM in colourimetric and electrochemical methods, respectively. The probe is successfully applied to estimate the levels of HSO 3 − ions in natural water and red wine samples. [Display omitted] [ABSTRACT FROM AUTHOR]