The invention provides a method of determining the amount of an analyte having an oxidation potential, for a one electron oxidation process, of about +0.10 to about +1.20 volts at pH 7, relative to the normal hydrogen electrode at 298K, said method comprising measuring the emission intensity or emission lifetime, at two or more wavelengths, from a sample comprising said analyte and two or more different macrocyclic lanthanide (III) complexes, wherein each of said macrocyclic lanthanide (III) complexes comprises a different lanthanide ion but the same macrocyclic ligand, and using a ratio of emission intensities or emission lifetimes measured at two different wavelengths to calculate the amount of analyte in said sample.