The selective assay of aluminum and cyanide ions is reported using fluorescence enhancement and quenching of a phenol–naphthol based chemosensor (PNI) in aqueous and nonaqueous solvents, respectively. PNI gave no significant fluorescence in water. The binding properties of PNI with metal ions were investigated by UV-vis, fluorescence, and electrospray ionization mass spectrometry in a Bis–Tris buffer solution. The addition of aluminum ions switches on the fluorescence of the sensor PNI in water, comparable to relatively very low fluorescence changes in the presence of various other metal ions. The complex stability constant (K_a) for the stoichiometric 1 : 1 complexation of PNI with aluminium ions was obtained by fluorimetric titrations and NMR experiments. However, upon treatment with cyanide ions, the fluorescence of PNI was selectively turned off and the yellow solution of PNI turned to red in methanol. Other comparable anions, such as F⁻, Cl⁻, Br⁻, I⁻, CH₃COO⁻, and H₂PO₄⁻, afforded no apparent fluorescence quenching. The interaction of PNI with cyanide ions was studied by NMR experiments.