Abstract The effect of the position of substituents on the formation of metal–naphthalene complexes has been investigated. Two positional isomers, 1-hydroxy-2-naphthoic acid (1H2NA) and 3-hydroxy-2-naphthoic acid (3H2NA), have been chosen. A comparative study of the luminescence behaviour of the two isomers in the presence of Zr(IV) has been performed. Interesting results were obtained. While 1-hydroxy-2-naphthoic acid is quenched in the presence of Zr(IV), 3-hydroxy-2-naphthoic acid produced high-fluorescence enhancement. Several pH studies were performed between pH 2.5 and 5.0 and the stoichiometries of the complexes were also established at the different pH values tested, by use of the Benesi–Hildebrand method. In addition, the formation constants have been calculated. Finally, quenching and lifetime studies were performed in an attempt to establish the type of quenching (static or dynamic) that is produced when a complex is formed between 1-hydroxy-2-naphthoic acid and zirconium metal ion.