This study investigated the treatability and changes in molecular weight distributions (MWD) of membrane concentrate. Nanofiltration (NF) concentrate from textile wastewater was subjected sole anaerobic treatment (R1) and pre-ozonation (R2) plus anaerobic treatment. BOD5, COD and TOC, color, sulphate and specific ultraviolet absorbtion at 254 nm (SUVA(254)) were analysed. Pre-ozonation improved the biodegradability of the membrane concentrate. The low molecular weight (LMW) organics was an important reason for the overall biodegradability enhancement of the membrane concentrate. Color was caused mainly by matters with higher than MWD of 10 kDa and 80% of HMW organics were removed during anaerobic treatment. Nearly half of sulfate was lower than 0.5 kDa which indicates that sulfate hardly treated by NF membranes. SUVA(254) values after anaerobic treatment were generally higher than 2 l mg(-1)m(-1) which indicates that membrane concentrate was in hydrophobic characteristics.