Pollution in five rivers (Aris, Belikas, Epis, Nedon and Pamissos) of Messinia was studied before (September 2008) and after (November, December 2008 and January 2009) olive oil production period in order to conduct a preliminary study on the effects of the olive oil wastes to the water bodies (rivers and marine coastal areas) of Messinia. The results obtained could be used as a useful tool in defining guidelines for olive oil producers, so as to reduce the amount of organic pollutants in the water ecosystems. Measurements of nutrients, total organic carbon (TOC), dissolved trace metals and total phenols were performed in the riverine waters and the seawater close to the mouth of the rivers together with measurements of major and minor elements, organic carbon, phenolic compounds and organic pollutants in sediment samples. Rather elevated amounts of phenols (1.8 – 7.9 mg/L) and high concentrations of ammonium and inorganic phosphorus (7.29 – 18.9 mmol/L of ammonium and 0.5 – 7.48 mmol/L of inorganic phosphorus) were measured in small streams located close to the various olive oil refineries of the area, which are directly influenced by the liquid waste disposals. This is an indication that the produced wastes contain high quantity of phenols, potentially dangerous for the environment and are also rich in inorganic nitrogen and phosphorus. However, significant reduction of inorganic nutrient and phenol content was recorded in the five main rivers studied, receiving the polluted waters of the small streams. Temporal and spatial variation of inorganic and organic pollutants was observed in riverine waters. In general, higher concentrations were found during the producing period. Pamissos and Aris rivers seem to be more enriched in dissolved metals and inorganic nutrients, whereas the highest TOC concentrations (> 7.5 ppm) were measured in Epis river during December. The dominant form of nitrogen in the five rivers before the producing period was nitrate (Nitrate/DIN was 78%), while during the producing period ammonia was the dominant form. Regarding river sediments, the highest concentrations of almost all the chemical parameters were recorded in Aris river, which seems to be the most polluted (organic carbon > 2%; phenols >0.8 μg/g; PAH>500 ng/g). Metal content in sediments was rather low, following a decreasing trend downstream. However, high Cr concentrations (> 200 ppm) were found in all the rivers except Nedon. In all the sediments examined a series of aliphatic aldehydes with more than 14 C atoms were detected. The presence of these compounds is not very common and their relation to the olive oil wastes will be further examined. Although those rivers are also influenced by other industrial activities, olive oil wastes seem to be a cargo which can potentially disturb the functioning of the ecosystems.