Lignin and tannin are widespread natural compounds traditionally used in tannery industries. Their presence is commonly detected in textile wastewater showing potential toxicity effects within various endpoints onto sea water organisms that generally represent the ultimate target of discharged effluents. Most data are available only as nominal concentrations or percentage volume of wastewater having an unknown lignin and tannin content. The aim of this study was to provide the ecotoxicological characterisation of both compounds considering as testing species the marine alga Phaeodactylum tricornutum (Bohlin). Lignin and tannin showed an E(r)C(50) of 113.84 (100.90-128.45) mg/L and 26.04(20.10-33.95) mg/L, respectively. NOEC and LOEC values were together <0.1 mg/L and 0.1 mg/L, in that order. Moreover, it was observed a morphological change of the algae fusiform shape occurring only at tannin concentrations >= 75 mg/L and <185 mg/L. (C) 2011 Elsevier B.V. All rights reserved.