This follow-up study tended to provide a systematic comparison for how the variation of functional groups and molecular structures present in model azo dyes affects color removal capability of Pseudomonas luteola. As sulfo group at methyl orange (p-MO) or carboxyl group at 4-(4'-dimethylaminophenylazobenzoic acid) sodium salt (denoted p-MR) were both para to azo bond, the ranking of decolorization rate was p-MO > p-MR due to the stronger electron-withdrawing effect of the sulfo group. For isomers, when the functional groups (sulfo, group at 2(4'-dimethylamino-phenylazo) benzenesulfonic acid sodium salt (o-MO) or carboxyl group at methyl red (o-MR)) were ortho to azo bond, the decolorization rate significantly decreased (e.g., p-MO > o-MO or p-MR>> o-MR) likely due to steric hindrance near azo, linkage(s). Similarly, for phenolic azo dyes the series of decolorization rate was 3-(4'-dimethylaminophenylazo) phenol (m-OH) > 2-(4'-dimethylaminophenylazo) phenol (o-OH). Apparently, azo dyes with different properties of substituent on aromatic ring could affect the efficiency of biodecolorization of P luteola. Moreover, the relative position (e.g., ortho, meta, para) of the substituent to azo bond could also influence the capability of biodecolorization. of P. luteola. Regarding the electronic effect, azo dyes with stronger electron-withdrawing group (e.g., sulfo group) at specific positions (e.g., at para) could be more easily biodecolored than those with a carboxyl group. (c) 2007 Elsevier B.V. All rights reserved.