Lignosulphonates are known as biologically-difficult-to-degrade, multi-purpose macromolecules that potentially may create toxicity problems in biological activated sludge treatment systems and/or in receiving water bodies. In the present experimental work a commercial lignosulfonate formulation (COD = 320 mg/g; BOD5 = 8 mg/g) frequently being used as an auxiliarly chemical in the cotton and polyester dyeing process was subjected to ozonation (approximate to 3 g O-3/CODo) at different pH (6 and 12) and ozone doses (1,400 and 5,300 mg/h) to improve its biodegradability and reduce its toxicity. Experimental findings have indicated that ozonation of lignosulphonates is a rather kinetically limited process. 20 min ozonation of lignosulfonate at a dose of 1,400 mg/h and an initial pH of 12 resulted in an 8-fold decrease in acute toxicity (EC50 = 3-4% v/v) towards the micralgae Phaedactylum tricornutum) and 50% reduction in its inert COD content (CODo = 600 mg/L). COD-based molecular size distribution indicated that the lignosulphonate formulation was cleaved to low-molecular-weight COD fractions after ozonation under the above mentioned conditions.