The effect of the cheese whey's (CW) addition on the fermentative decolorization of olive mill wastewater (OMW) by Lactobacillus paracasei, with and without pH adjustment by lime, was investigated. Mixtures OMW/CW at differentiated proportions were fermented. The highest colour removal (47%) and total phenolic reduction (22.7%) of OMW were obtained after cofermentation of OMW/CW at proportions of 10/90, respectively. The decrease of pH after cofermentation of the two wastewaters, induced the precipitation of whey proteins with phenolic compounds and, so, improves decolorization. These removal yields reached 64% and 34%, respectively after precipitation by adjustment of pH at 7 with lime at the end of cofermentation. These improvements were correlated to a clarification of wastewaters by precipitation of whey proteins with phenolic compounds. An enhanced decolorization (up to 93%) and a total phenolic reduction (50%) of the mixture were obtained when cofermentation sequentially pH corrected by lime addition was investigated. (C) 2009 Elseiver Ltd. All rights reserved.