Sorption of acetic, propionic, butyric, valeric, and isovaleric acid on chitosan and cross-linked chitosan was studied in aqueous solutions and water/ethanol mixtures. Langmuir, Freundlich, and Redlich-Peterson equations were used to fit experimental isotherms of sorption. Correlation between maximum sorption and the pK value of every carboxylic acid was found for sorption on chitosan in aqueous solutions, while sorption in water/ethanol mixtures was mainly determined by structure and the hydrocarbon chain length of the carboxylic acid. Slight molecular-sieves effect was found for carboxylic acids sorption on cross-linked chitosan.