Thermodynamic calculations of the solubility of Pt and Pd up to 300 degrees C using isocoulombic extrapolation, the correspondence principle, linear free energy relationships, and the theory of stepwise ligand replacement indicate that, depending on physicochemical conditions, complexation by either OH (super -) , HS (super -) , or Cl (super -) can contribute to the transport of these metals in hydrothermal solutions.The soft character (in the Pearson sense) of Pt (super +2) and Pd (super +2) dictate that complexes of these ions with CO (super -2) 3 , HCO (super -) 3 , SO (super -2) 4 , PO (super -3) 4 , and F (super -) will be weak. Chloride complexing will be important only under highly oxidizing and acidic conditions. In highly saline fluids (i.e., Sigma Cl (super -) > 1.0 m), the predominant chloride complexes of Pt and Pd are PtCl (super -2) 4 and PdCl (super -2) 4 from 25 degrees to at least 300 degrees C. Approximately 10 ppb or greater of either Pt or Pd may be transported as chloride complexes in solutions in equilibrium with hematite at low pH (<3 for Pt and <4 for Pd) at 300 degrees C. Hydroxy complexes become important at slightly acidic to neutral pH and may be responsible for Pt and Pd concentrations of 1 to 10 ppb near the pyrite + pyrrhotite + magnetite triple point (i.e., near-neutral, relatively reducing conditions). The soft nature of the Pd (super +2) and Pt (super +2) ions suggests that bisulfide complexing should be important, in analogy with Au. At 300 degrees C and Sigma S = 0.1 m, it appears that Pt(HS) (super -2) 4 and Pd(HS) (super -2) 4 concentrations of at least 10 ppt may be attained near the pyrite + pyrrhotite + magnetite triple point. At Sigma S = 0.5 m, solubilities increase to 1 ppb in the same region. Under the conditions where solubility as HS (super -) complexes reaches a maximum at 300 degrees C, solubility as Cl (super -) is negligible. Between 25 degrees to 250 degrees C, solubility as HS (super -) complexes increases with temperature while solubility as Cl (super -) or OH (super -) decreases. The trend reverses above 250 degrees C. The effect of Se, Te, and As is generally to decrease the mobility of Pt and Pd, although the possibility of complexing by ligands involving Se, Te, and As cannot be entirely discounted.These calculations suggest that Pt and Pd may be mobile in a variety of environments. In kupferschiefer-type ore-forming fluids they could be transported as chloride complexes. The lack of acidic alteration and oxidized mineral assemblages in hydrothermal Pt and Pd occurrences in shear zones in metagabbroic rocks, such as those at New Rambler, Wyoming, and Rathbun Lake, Ontario, preclude Pt and Pd transport as chloride complexes during formation of these deposits. In these cases, bisulfide and/or hydroxide complexes are more likely responsible for precious metal transport.