Many coastal areas in the Gulf experience a rapid industrial and urban growth. This development is sustained by an increasing number of seawater desalination plants in the region which satisfy the growing demand for fresh water. The combined seawater desalination capacity in the Gulf countries exceeds 11 million cubic metres per day and accounts for 45% of the total world capacity. The predominating process is multi-stage flash distillation (MSF), whereas only a minor amount of the drinking water is produced by reverse osmosis (RO) plants and other processes (together < 15%). Due to their waste water discharges to the sea, desalination plants - and especially MSF plants - must be considered a main source of pollution in the Gulf. It is estimated that the combined discharge of all MSF plants in the Gulf amounts to a waste water flow of about 1,000 m
per second - which is the equivalent of a major river. This waste water is characterised by increased salinity and elevated temperature. It additionally contains substantial amounts of chemical pollutants, such as chlorine (which is used for biofouling control in the plants), antiscalants (which are used for scale inhibition) and heavy metals (which are present due to corrosion). This paper gives an overview on the waste water characteristics of the two main desalination processes (MSF, RO), presents estimates of total chemical discharges to the Gulf for two selected pollutants (chlorine, copper) and discusses the potential impacts of seawater desalination activity on the Gulf’s marine environment.