Experimental and calculated complex formation curves (ECFC and CCFC) for a labile metal-ligand system are defined and used for the speciation study by differential pulse polarography (DPP) at fixed total ligand (L-T) to total metal (M-T) concentration ratio and varied pH. The CCFC is used for the modeling of species formed in a solution and optimization of formation constants for these species. The CCFC can be generated for any metal-ligand model, including polynuclear metal species, for any L-T:M-T ratio, and for more than one ligand competing in the complex formation reaction. The metal-ligand model and formation constants are optimized by solving mass-balance equations written for the assumed model and by fitting the CCFC into the ECFC. The methodology of building up the metal-ligand model with initial values for the formation constants, used for further refinement, is described. Lead(ll) complexes with the ligand N,N,N’,N’-tetramethylethylenediamine (TMen) are studied by DPP, at 298 K and mu=0.5 mol l(-1), at large L-T:M-T ratios and varied pH. Three lead complexes with the ligand TMen, viz. PbL2+, pbL(2)(2+) and PbL2(OH)(+), were found and their overall stability constants, as log beta, were estimated to be 2.56+/-0.03, 4.86+/-0.03 and 9.93+/-0.02 respectively (all known hydroxide species of lead, including polynuclear species, were incorporated into the Pb-TMen-OH system). The results obtained for Pb-TMen seem to be reasonable when compared with those for Pb-ethylenediamine reported previously [I. Cukrowski et al., Polyhedron 14 (1995) 1661].