Electrochemical techniques (two different polarographic techniques and the ion specific electrode method) were used to measure the equilibrium constants of lead(II) organic complexes up to 90 degrees C. The data are extrapolated to 200 degrees C. The equilibrium speciation model EQBRM was used to calculate the significance of organic complexing in Na-Ca-Cl-bearing brines, from 50 degrees to 200 degrees C, by comparing total organic complexing to total chloro complexing with lead(II) and zinc(II). The significance of organic complexing in hydrothermal brines depends mostly on the solubility of the calcium(II) organic salts, the concentration of calcium in solution, the thermal stability of the organic species, and the acidity of the ligands' binding groups. The model showed that acetate (or other soluble carboxylates) complexes with lead(II) and zinc(II) can predominate over chloro complexes under neutral to slightly acidic pH conditions when present at concentrations which have been observed in hydrothermal brines. A minimum concentration of about 10 (super -3) m of organic ligand is necessary for species such as oxalate to complex base metals in Na-Ca-Cl-rich brines in a significant way. This concentration (10 (super -3) m) is one or more orders of magnitude supersaturated with respect to Ca oxalate. The low calcium salt solubility also prevents ligands, such as phthalate, anthranilate, picolinate, dipicolinate, 1,10 phenanthroline, and thiosalicylate, to be of potential importance for base metal transport in Ca-rich hydrothermal fluids.