Biothiols such as Cysteine (Cys) and hydrogen sulfide (H₂S) undergo metabolism to generate sulfur dioxide (SO₂) as the end-product, which spontaneously forms bisulfite (HSO₃^-) in physiological systems. In contrary, sulfur dioxide derivatives are health hazards and act as environmental pollutant. Excessive intake of SO₂, which ultimately produces bisulfite, may cause several critical health complications including respiratory diseases and even lung cancer.1 Accordingly, it is utmost necessary to develop an effective detection method for bisulfite to investigate the biothiol metabolism pathway as well as SO₂/HSO₃^- induced health disorders. In our continuous efforts to develop fluorescent probe for redox related species, this time we introduced a two-photon (TP) active fluorescent probe, which developed for the selective detection of bisulfite ions. The probe is based on a benzocoumarin (BC) dye containing an aldehyde moiety, which shows ratiometric fluorescence response (emission shift = 63 nm) in presence of bisulfite. Furthermore, this probe is also effective for ratiometric imaging of endogenous bisulfite ions in HeLa cells under two-photon excitation. References: 1. Sang, N.; Yun, Y.; Yao, G.; Li, H.; Guo, L.; Li, G. Toxicol. Sci. 2011, 124, 400−413. *Unpublished data