The No. 1 desalination plant of the Al-Ghubrah power and desalination plant, Sultanate of Oman, is a multi-stage flash long-tube parallel flow type with 18 stages of heat recovery and two stages of heat reject in five vessels. The plant was commissioned in the year 1976. It had experienced severe corrosion in the vapour space, and more in the high temperature stages. The support plate, inter-stage walls, and top roof plate were badly corroded. Tube bundles began to sag and some collapsed in vessel No. 1. Tube leaks were encountered frequently. It was analyzed that corrosion was due to build-up of non-condensable gases in these stages. The plant venting system was designed to have series venting (cascaded) from stage No. I to 20. The plant was rehabilitated in 1990. The paper describes the study of corrosion, modifications of the venting system and preventive measures being carried out to prevent the accumulation of non-condensable gases. After modification and tube bundle replacement in vessel No. 1, no significant corrosion has been observed in this vessel even after 5 years of operation.