Treatment of mono-azo dye, tectilon yellow 2G (TY2G), by Chlorella vulgaris was investigated. COD removal efficiencies were determined as 69, 66 and 63% for the initial TY2G concentrations of 50, 200 and 400 mg/l, respectively, whereas acclimation of C vulgaris caused them to increase to 88, 87 and 88%, respectively. Absorbance spectral profiles obtained for unacclimated algae showed that the peak observed initially at 450 nm disappeared and the one at 220 nm decreased remarkably while there was a new peak formation at 350 nm, indicating the conversion of TY2G to an end product which was further confirmed as aniline by high-performance liquid chromatographic analysis. In the case of acclimated algae, no aniline production was detected. The main mechanism of the removal was bioconversion in the case of unacclimated algae and degradation in the case of acclimated algae. Moreover, the higher the initial algal concentration, the higher is the COD removal efficiency achieved in a much shorter time. (C) 2003 Elsevier Ltd. All rights reserved.