Two fluorescence techniques to study metal-humic interactions are presented. In the first technique, Lanthanide Ion Probe Spectroscopy (LIPS), the humic samples are titrated by Eu(+3) ions. The ratio of the intensities of two emission lines of Eu(+3), R=I(sub 592)/I(sub 616), is used to estimate the amount of bound and free species of the probe ions. The titration plot is presented as R versus the logarithm of total added Eu(+3). In the second technique, fluorescence quenching of the humic material by Cu(+2) is used to produce titration curves of intensity versus the logarithm of total added Cu(+2). The two techniques are used in conjunction with a model that treats the various ligands in humic substances as continuous distributions of binding sites in which individual ligand concentrations are normally distributed with respect to the individual stability constants for metal binding. The model includes the effects of pH, ionic strength, and competing metal ions. The parameters of the model are estimated by fitting the spectral titration data to the calculated titration plot. Some simulation and experimental data are presented and discussed. (Copyright (c) Springer-Verlag 1991.)