The effectiveness of an Eu3+ luminescence technique to describe metal complexation reactions in humic substances was evaluated. The molecular uncertainties intrinsic to humic substances were circumvented by using polyacrylic acid (PAA) and polyvinylsulfonic acid (PVS) as probes. An adequate determination of the Eu3+-citrate affinity constant suggested that this technique could potentially be used to characterize metal binding reactions in polyelectrolytes. Various mixtures of PAA and PVS were then evaluated. The commonly adopted assumption of a single metal binding environment in those systems proved ineffective and resulted in a misleading representation of the Eu-PAA binding isotherm. Our evidence indicates that prior knowledge about the number of binding environments, as well as their relative contribution to the overall sorption, is needed to describe metal binding on humic substances appropriately through this approach. The effect of metal competition on Eu3+ binding by a mixed polymer system (1:1 PAA:PVS) was also ascertained. The affinity series obtained (Cu2+ > Pb2+ much greater than Cd2+ greater than or equal to Co2+ congruent to Mg2+ congruent to Ni2+ > Ca2+) was similar to that frequently observed in humic substances, which reinforces the utility of these polymers as their surrogates in metal sorption studies.