A fluorescent molecule (phi = 0.32) comprising two benzimidazole fluorophores condensed with a N, S macrocycle with sufficient solubility in water (10(-4) M) has been studied as sensor for a series of metal ion aqueous solutions (including Cr3+, Fe3+, Co2+, Ni2+, CU2+, Zn2+, Hg2+ and Pb2+). The complex formation constant for dipositive cations in water is low (similar to 0.2 M-1). but it is remarkably larger for tripositive cations, particularly for Cr3+ (100.2 M-1). This indicates that the main factor controlling complexation is the Lewis acidity of the cation and its interaction with hard N atoms, The emission is quenched by the presence of the above metal ions at a diffusion control quenching rate.