Recently, new processes for recovery of monocarboxylic acids have been studied. Among these processes, liquid-liquid extraction has had special attention. This process of extraction is attractive if an adequate solvent is used, which shows high selectivity and efficiency, does not affect the thermal stability of products, and requires low energy consumption. In the present work, extraction equilibria of the systems tributyl phosphate and trioctylamine in organic diluents (decan-1-ol and dodecane), water, and each of five lower monocarboxylic acids were measured at the temperature (20 +/- 1) degreesC. For each studied acid are obtained different extraction isotherms in dependence on the composition of the organic phase. From the treatment of experimental data it is shown that the estimated values of the equilibrium extraction constant and the apparent number of extractant molecules reacting with one molecule of monoacid depend not only on the organic phase composition but also on how they are determined.