Many methods have been employed for measuring the interactions of trace metals and humic materials but little consideration has been given to the magnitude of error in the resulting titration data. Most common metal-titration methods can be placed into one of three categories based on the chemical species that is most directly measured: free ionic metal, total labile metal, or ligand-bound metal. The propagation of random instrument error into a titration data set is not uniform throughout a titration and the pattern of propagation is shown to be characteristic of the category of method used. Standard deviations are determined for measurements by ion-selective electrodes, fluorescence quenching, and fixed potential amperometry (each representative of a category of methods). Error propagation in resulting titration data is quantified. No single titration method yields uniformly precise data in all concentration ranges and a combination of complementary methods yields the best characterization of metal binding to a humic material.