The effect of olive mill wastewater (OMW) phenolic compounds with low and high molecular weight depolymerisation by Lactobacillus plantarum on the transport of phenolics from OMW to olive oil was studied. Incubation of olive oil samples with fermented OMW by L. plantarum caused polyphenols to decrease in OMW and increase in oil with multiple biological effects. The lower total phenolic content in fermented OMW of 845 mg/l in comparison to OMW control with 1247 mg/l resulted from the depolymerization of phenolic compounds of high molecular weight by L. plantartum. Fermentation with L. plantarum induced reductive depolymerisation of phenolic compounds of OMW which are more soluble in olive oil. The total simple polyphenols content of olive oil mixed with OMW and fermented by L. plantartum was higher (703 mg/l) than an olive oil control mixed with OMW (112 mg/l). Simple polyphenols content was increased in olive oil when L. plantarum was added to OMW, especially for oleuropein, p-hydroxyphenylacetic, vanillic and ferulic acids and tyrosol. (C) 2003 Elsevier Ltd. All rights reserved.