Tertiary amines dissolved in organic solvents are effective chemical extractants for the recovery of carboxylic acids from aqueous phases. In this series of publications experimental results for the liquid-liquid equilibrium in organic/aqueous two-phase systems containing a tertiary amine (tri-n-octylamine) and two carboxylic acids (acetic, oxalic and citric acid, respectively) are presented and compared to model predictions. This first part deals with toluene as an organic solvent. Experimental results are reported for 295.15 K. A model for the liquid-liquid equilibrium, which in both phases takes into account chemical reactions as well as physical interactions, is used to predict the liquid-liquid equilibrium when two carboxylic acids partition between the aqueous and organic phases. All model parameters were determined from experimental results for subsystems including the partitioning of the single carboxylic acids onto water/(toluene + tri-n-octylamine) phases. The model predictions agree qualitatively with experimental results. When a mixture of citric and oxalic acid is extracted, the predicted partitioning of the acids agrees also quantitatively with the experimental results. However, when acetic acid is extracted together with citric or oxalic acid, experimentally determined distribution ratios of acetic acid are higher than predicted. Quantitative agreement is achieved by assuming the formation of mixed acid complexes (i.e. complexes containing two different acids).