Over the past fifteen years the Commission on Equilibrium Data of the Analytical Division of the I nter­ national Union of Pure and Applied Chemistry has been sponsoring a noncritical compilation of metal complex formation constants and related equilibrium constants. This work was extensive in scope and resulted in the publication of two large volumes of Stability Constants by the Chemical Society (London). The first volume, edited by L. G. Si"en (for inorganic ligands) and by A. E. Marte" (for organic ligands), was published in 1964 and covered the literature through 1962. The second volume, subtitled Supplement No.1, edited by L. G. Si"en and E. Hogfeldt (for inorganic ligands) and by A. E. Marte" and R. M. Smith (for organic ligands), was published in 1971 and covered the literature up to 1969. These two large compilations attempted to cover a" papers in the field related to metal complex equilibria (heats, entropies, and free energies). Since it was the policy of the Commission during that period to avoid decisions concerning the quality and reliability of the published work, the compilation would frequently contain from ten to twenty values for a single equilibrium constant. In many cases the values would differ by one or even two orders of magnitude, thus frustrating readers who wanted to use the data without doing the extensive literature study necessary to determine the correct value of the constant in question.