Using a novel concentration technique (reverse osmosis and freeze-drying) as well as a standard analytical technique little used with limnological samples (solid state 13 C nuclear magnetic resonance), we studied the chemical structure of aquatic organic matter from four closely located freshwater sites in Nova Scotia . The main conclusions drawn from the data are that : (a) the aromatic C fraction which is assumed to be refractory remains at less than 10% of the total, with a slight increase in relative importance in the fall, (b) less structurally complex aliphatic carbon decreases from winter to spring and remains at lower levels into later fall, (c) carbohydrates are at a maximum during the summer, (d) the carboxylic C fraction is at a minimum in the summer and maximum in the fall and winter . Results show roughly the same annual patterns of C composition for the two running water sites . Our data suggest that the hydrological processes which carry terrestrial and wetland DOM to streams and lakes allow a selective adsorption process of DOM to occur in soils . We compare our data to those from other freshwater studies and suggest that the importance of aromatic carbon in freshwaters has probably been overestimated in the past, with a corresponding underestimate of the more biologically labile carbohydrate and aliphatic material .