In the course of water deionization on ion-exchange filters of mixed action, charged with a mixture of KU-2×8 ch.s. cation exchanger and AV-17×8 ch.s. anion exchanger, which are ion-exchange resins of nuclear class, the water being treated is contaminated with soluble organic substances originating from degradation of the resins. Their concentration can reach 20 μg L–1, gradually decreasing to 5–6 μg L–1. Procedures were suggested for determining soluble organic substances washed out from the ion-exchange filters, including gravimetric determination of their sum in aqueous extracts from the organic ion exchangers, spectrophotometric determination in water being purified on the ion exchangers, and preconcentration of electrically charged forms of soluble organic substances by electroosmosis.