A computer program (MODELm) is described for calculating the trace metal speciation of mixtures of several metals and natural organic matter by using a linear differential equilibrium function model that takes into account the metal binding dependence on the metal : complexant concentration ratio. MODELm was able to predict the degree of metal ion complexation by humic acid over a range of metal ion loadings in good agreement with reported experimental data. When compared with the widely used speciation code MINTEQA2, MODELm predicted a much greater amount of metal–NOM complexation under given conditions, and a marked dependence of the extent of metal–NOM complexation on the concentration of NOM. These results have significance for metal ion speciation modelling in natural systems.