The “Addition–NGP (neighbouring group participation)–Elimination” technique has been used for the ratiometric sensing of H₂S/HS⁻. We report herein the sensing of H₂S by the nucleophilic addition of H₂S/HS⁻ to a –CHO group of R1 adjacent to naphthyl hydroxyl followed by NGP of thiol through a probable six-membered transition state that undergoes the cleavage of the dinitrophenyl ether moiety. In contrast, R2 undergoes a probable addition and elimination mechanism forming a Meisenheimer complex, which also cleaves the ether moiety. However, herein, the reaction is very slow compared to R1, which works through NGP. The probe can be highly promising for selectivity and sensitivity, to detect H₂S/HS⁻ with rapid response (<3 min) and it opens a new way for the thiolysis of the ortho-hydroxy aldehyde platform by cleavage of an active ether linkage. Test strips based on R1 were fabricated, which could act as a convenient and efficient H₂S/HS⁻ test kit to detect H₂S/HS⁻ in an aqueous solution and in a solid phase for practical application.