Abstract: The mechanism of extracting acids from water solution with trioctylamine was studied, It was found that the mechanism of extracting strong acids is different from that of extracting weak acids; diluent effect in the latter case is much stronger.The extractive capacity of weak acids with trioctylamine depends on the interaction between the extraction complex and the diluent as well as the solvophobic properties of the diluent; the stronger the interaction and the weaker the solvophobic properties of the latter, the better the extraction of carboxylic acids.