A new mechanism for the fluorescence detection of metal cations in solution is introduced involving a unique keto–enol tautomerization. Reduction of 1,8-anthraquinone-18-crown-5 yields the doubly reduced secondary alcohol, 2. Compound 2 acts as a chemodosimeter for Al(III) ions producing a strong blue emission due to the formation of the anthracene fluorophore, 3, via dehydration of the internal secondary alcohol in DMSO/aqueous solution. The enol form is not the most thermodynamically stable form under these conditions however and slowly converts to the keto form 4. Reduction of 1 with Fe/AcOH or the reaction of 2 with HCl directly yields compound 4, the keto tautomer of 3, which also produces the same blue emission in more polar solvents. Competition studies reveal that compound 2 produces a blue emission exclusively in the presence of the strong Lewis acidic Al(III) ion and at relatively low pH.