A multimode ratiometric Hg²⁺ selective chemosensor system based on a carbazole–thiobarbituric acid conjugate (CTBA) was developed. Its colorimetric and fluorescence behavior towards Hg²⁺ ions in both homogeneous and microheterogeneous media were investigated. A ratiometric mode of response of absorption as well as fluorescence in acetonitrile and 9 : 1 THF/H₂O medium was observed in the presence of Hg²⁺ ions. The sensing mechanism involves formation of a 1 : 2 CTBA : Hg²⁺ complex by deprotonation of imide protons of the thiobarbituric acid unit. The complex formation can be reversed by using competing ligands such as cysteamine hydrochloride.