The use of tangential flow filtration (TFF) for size fractionation of natural dissolved organic matter was investigated. The performance of regenerated cellulose menbrane with a nominal molecular weight cut-off of 1kDa was examinated on 20 samples from lake, river and estuary systems, characterised by contrasting dissolved organic carbon (DOC) contents and conductivity. The evaluation was based on absordance, fluorescence and DOC measurements. Detailed protocols of membrane cleaning and conditioning nation are proposed. The ultrafiltration membrane can efficiently be cleaned to provide low carbon blank (<0.01mg/L). Fluorescence measurements confirmed that the higher molecular weight compounds were isolated in the retentate and the lower molecular weight remain in the permeate. Mass balance for natural samples show good recovery for DOC (109 ± 12%, n= 20) and fluorescence measurements (106 ± 9%, n=13). No relation between factors of concentration (fc) and mass balance quality was observed for the fc range 1.5 - 11. Moreover, high ionic strength and high DOC contents did not enhance membrane fouling. The findings demonstrate that reliable fractionations by TFF of natural organic colloids in aquatic systems can be achieved.