Complexation of three 3-hydroxychromone derivatives bearing a nitrogen-containing heterocyclic moiety in the position 2 of the chromone bicycle - benzimidazole, quinoline, and 2,5-diphenyloxazole, with mercury(II) ions is reported. Formation of chelate complexes with the metal cations coordinated with the cavity formed by 3-OH and 4-C=O groups was shown, as well as the possibility of side moiety heteroatom participation in binding of metal ions. High sensitivity to mercury of 2,5-diphenyloxazole-substituted 3-hydroxychromone was elucidated, allowing to detect Hg2+ below the maximum permissible concentration for drinking water. This makes the above-mentioned compound a prospective basis for development of sensors for ultralow mercury concentration detection in water. Unusual fluorescence ignition of 2-(quinolin-2-yl)-3-hydroxychromone at low Hg2+ concentrations, rarely observed for heavy metals ions complexation with organic fluorescent ligands, was discussed.