Equilibrium distribution ratios have been determined for the extraction of cobalt(II) with 8-quinolinol and its 2-methyl-, 4-methyl-, 5-chloro-, and 5-nitro- analogues into chloroform as a function of pH and reagent concentration at ambient temperature. Except in the case of 2-methyl-8-quinolinol, which formed a simple 1:2 chelate, the extractable complexes were mono-adducts, that is, 1:2 chelates to which a third molecule of the reagent was coordinated. The overall formation constants of the cobalt chelates in aqueous solution were determined. A linear correlationship was observed between the formation constants, adduct formation constants, and the basicity of the reagent. This dependence could be used to predict, based on known data, the stabilities of not yet studied complexes of cobalt with this family of ligands. The distribution coefficients of the chelates and the chelate adducts are discussed in terms of structural influences. A penta-coordinate, square pyramidal structure has been proposed for the self-adducts of cobalt(II) with 8-quinolinols (except 2-methyl analogue).