This study investigates inert COD levels and toxicity of two different commonly applied textile dye auxiliaries, namely two tannin formulations (Tannin 1: a condensation product of aryl sulphonate; Tannin 2: a sulfonated phenol polymer). The experimental evaluation is performed on samples obtained by simulating the textile bath discharges carrying the tannin formulations. The acute toxicity of tannins is assessed by employing the photobacterium Vibrio fischeri. The results obtained indicated that both of the tannin formulations have high levels of initially inert soluble COD, SI, accounting for 59 and 41 % of the total COD, respectively. Such an outcome necessitates a specific pretreatment on segregated effluents containing these formulations. Acute toxicity data showed that both of the formulations are toxic towards Vibrio fischeri, although Tannin 2 exerts considerably higher toxicity.