It Is shown that Immobilization of metal chelating photometric reagents can have the effect of Increasing their selectivity. The ligands studied , 3-(2-pyrldyl)-5,6-bls(4-sulfonatophenyl)-1,2,4-trlazlne and 2 ,4-bls(5,6-bls(4-sulfonato-phenyl)-1,2,4-trlazln-3-yl)pyridlne, become selective for Cu+ over Fe2+ upon Immobilization. It Is argued that the selectivity results from the Inability of the 3:1 (llgand-to-metal) Fe2+ complexes to form due to restrictions Imposed by reagentsubstrate interactions. The Cu+ complex Is affected to a lesser extent due to the less demanding 2:1 stoichiometry.