The interphase distribution of weak acids in systems involving salts of quaternary ammonium bases with organic acids (binary extractants) has been analyzed. Significant differences in the extraction depending on the composition of the aqueous and organic phases for weak acids compared with that for strong acids in binary extractant systems have been shown. The difference in the extraction of weak acids with binary extractants compared with neutral extractants has been also revealed. The experimental data were interpreted by two variants of distribution processes, when two extracted species are formed (R4NX and HA) and for the case of forming extracted species with strong H-bonds (R4NX center dot HA). The least square regression method showed that the proposed mechanisms well describe the extraction of monocarboxylic acids with the binary extractant (the smallest sum of squares of relative deviations was 0.025) at their low initial concentrations (<0.05mol center dot L-1). It was shown that the most effective extraction of monocarboxylic acids takes place from acidic solutions (at pH<2) that indicates the possibility of their effective stripping from organic solutions with water.