The technique of diffusive gradients in thin films (DGT) was used to investigate the distribution of Pb between inorganic forms and organic complexes in various solutions that contained fulvic acid, humic acid, or nitrilotriacetic acid (NTA) over the pH range 4–8. Three types of DGT devices with diffusive gels of different pore sizes were used. When the diffusion coefficient of each species in each gel type was considered, the DGT measurements obtained agreed well with the distribution of species predicted by the ECOSAT equilibrium speciation model for all solutions and gel types. When an appreciable proportion of inorganic species was present in solution, direct measurements by DGT using the most restricted gel provided a reasonable estimate of the inorganic species in solution. This demonstrates that DGT is able to use differences in molecule mobility to distinguish between species in solution.