The molecular size distribution of total organic carbon in a secondary textile/domestic effluent and in tertiary effluents (multi-media filtration, flocculation + parallel plate settling and granular activated carbon at pilot scale) has been determined by membrane separation (3 x 10(3) MWCO to 0.22 mu m cut-offs). Cross-flow and dead end filtration schemes were used. The removal efficiency related to each dimensional class provides useful information for the choice of the most suitable treatment, or sequence of treatments, depending on the performance required. It appeared that both flocculation and GAC were effective in removing organics smaller than typically reported in the literature. Because of the molecular size distribution of the effluent fed to the polishing post-treatments, it was found that filtration by a 3 x 10(5) MWCO cut-off microfiltration membrane would provide a 50% TOC removal, a performance well above multimedia filtration or flocculation + parallel plate settling.