The new heteroditopic ligand 2,3-dihydroxy-N-(1,10-phenanthroline-5-yl)benzamide (H2-L3) was synthesized and coordinated to [Ru(bpy)2(phen)]2+- and [ReBr(CO)3(phen)]-type luminophores (bpy = 2,2'-bipyridine and phen = 1,10-phenanthroline). The resulting chemosensors [Ru(bpy)2(H2-L3)]2+ and [ReBr(CO)3(H2-L3)] were fully characterized and their solid-state structures and spectroscopic properties were investigated to assess how the photophysical properties of the luminescent signaling units affect the performance of the sensors. [Ru(bpy)2(H2-L3)]2+ and [ReBr(CO)3(H2-L3)] both signal the presence and concentration of molybdate and vanadate in aqueous acetonitrile through a decrease in emission intensity. [ReBr(CO)3(H2-L3)] also detects tungstate. Due to the higher emission intensity of the Ru-based sensor, its detection limits for molybdate (43 microg L(-1)) and vanadate (24 microg L(-1)) are almost 1 order of magnitude lower than the ones achieved with the Re-based sensor. The optimum working pH of the chemosensors is determined by the pKa values of the 2-hydroxy-groups of the receptor units: pH 4 for [ReBr(CO)3(H2-L3)] and pH 3 for [Ru(bpy)2(H2-L3)]2+. Both sensors are selective: equimolar amounts of PO4(3-), SO4(2-), ReO4-, Mn(II), Fe(III), Co(II), Ni(II), Cu(II), and Zn(II) do not interfere with the detection of molybdate or vanadate.