The purpose of this work is to study the extractive capacity of an organophosphorus compound to recover carboxylic acids from the waste water resulting from the ozonation process for the bleaching of pulp in order to reduce the environment pollution. In this work, the selected solvent was composed of a mixture of tributyl phosphate (60 vol %) with dodecane (40 vol %) and the solutes under consideration were acetic, formic, and oxalic acids. With no exception, it is the undissociated acid that is extracted by the solvent. Ternary system solubility diagrams water + carboxylic acid + tributyl phosphate and tie-line data were established at the temperature of 25 degrees C. In comparison with the conventional extraction solvents such as alcohols, ketones, or ethers, which involve the solvation of the acid by donor bonds, the more basic donor properties of phophorus-bonded oxygen compounds show the stronger extractive capacity of this group of extractants. Furthermore, the carbon-bonded oxygen solvents are much more soluble in water and hence involve a costly solvent recovery. With regard to the tertiary amine extractants, which have low residual solubilities in water and high extractive power for several acids, the selectivity of organophosphorus compounds with respect to an individual carboxylic acid can be considered as an attractive property of this category of solvent. In this study, partition coefficients between 0.27 and 1.25 were determined for the separate acids.