During a transfer operation from a barge in an upper tier Seveso chemical site, 60 m³ of orthoxylene spilt onto the barge. The spill lasted for 45 minutes and polluted the OISE river. The operator reported the presence of a 300 m long hydrocarbon layer on the water surface. The internal contingency plan of the site was triggered. The inspection of the facilities classified, navigation department, local authorities and health authorities were informed. Air samples were taken to detect any possible formation of an explosive atmosphere. The measured xylene concentration near the river banks was in the order of 200 ppm (LEL 10 000 ppm). Four oil booms were installed downstream to the spill: two installed by the operator near the plant, two by the fire-fighters at 5 and 10 kms. However, they could not recover the products due to dilution and volatile nature of xylene. No death of fish was reported. The residents and especially local farmers were warned. They were advised against using the river water for animals. The infiltration of pollutants from the river bank into the river caused pollution to resurge on the following day at the site of the spill. An oil boom, pumping device, and site cleanup operations will be required to clean the soil polluted by xylene. The accident was caused due to a missing stop on the inspection flange of the transfer pipe: It was the 1st transfer operation after the pipe was de-commissioned for maintenance. While inspecting the pipe with a camera, the stop on the inspection flange has been removed and not re-assembled. This operation was carried out by a subcontractor who had only received verbal instructions from the operator instead of a concise written procedure. This oversight was not detected even during the inspections carried out upon site completion. The inspection authorities made note of the facts. The operator must modify the transfer facilities to detect leaks in transfer pipes and improve the safety management system.