The solubilities of the eight water insoluble substances in aqueous sodium benzoates solutions were determined. As a result of these measurement, it was found that any water insoluble substances were solubilized by sodium benzoate derivatives. Using the formation constants of the complexes, which were obtained by spectrophotometric measurements, the amounts of the complex in the water insoluble substance solubilized by sodium benzoate could be obtained. These amounts were found to be rather smaller than the solubility increase of the water insoluble substances. So, it was concluded that the other factor than complex formation should be considered as the mechanism of solubilization by sodium benzoate derivatives.