Kraft mill effluents were biologically treated in an aerated lagoon (AL) (3.4 L) to evaluate the behavior of aromatic compounds by UV-vis spectroscopy. Under aerobic conditions, biological oxygen demand (BODS) removal was between 84% and 95%, whereas soluble chemical oxygen demand (CODS) removal ranged between 40% and 60%. A slight increase was observed in the VIS436/CODS relationship (0.101-0.110), indicating that color reduction was less than the total organic matter reduction expressed by CODS. Additionally, low values of UV254/UV280 (1.23-1.12) indicate the presence of lignin-derived compounds in the aerobic effluent relationship. Ultrafiltration (UF) analysis shows that color was concentrated in the molecular weight fraction larger than 10 000 Da. The recalcitrant fraction was found to be resistant to further biodegradation, even under optimized microbiological conditions.