[en] The dissolved organic matter ''pool'' in a soft-water lake was fractionated into six molecular weight components using ultramembrane filtration. A statistical relationship was found between organic carbon concentrations and molecular weight ranges within individual size-fractions. In characterizing these fractions, high (> 100000-50000) and low (2000-0) molecular weight organic materials were found to be labile (about 50 percent decomposed in 4 h), following exposure to UV-light, while intermediate size fractions (50000-2000) were essentially refractory. Chelation-complexation capacity for inorganic iron (⁵⁹Fe) by high and low molecular weight fractions was increased significantly following exposure to UV-light. Bacterial ¹⁴C-labelled extracellular products were found to decompose at different rates in comparison to ¹²C organic compounds in individual molecular weight fractions, following exposure to UV-light, suggesting they were qualitatively different from one another. (author)