Shallow submarine hydrothermal fluids from Paleohori Bay off the Greek Island of Milos have been investigated in detail. The extensive gas and warm (max. 93 degrees C) water vents are enriched in major components such as Cl (super -) , Na (super +) , Si, Ca (super 2+) , Mn, PO 4 (super 3-) , NH (super 4+) , K (super +) , Sr (super 2+) , H 2 S and CH 4 and depleted in Mg (super 2+) , O 2 and SO 4 (super 2-) relative to ambient seawater. Based on zero-Mg calculations, it is deduced that the submarine vent fluids are related to the geothermal reservoir beneath Milos which has a similar chemical composition. Fluid samples from each vent in Paleohori Bay represent a mixture of hydrothermal end-member fluids derived from the upper and deeper geothermal reservoir systems of Milos, a mantle-derived source, a high-chloride fluid resulting from phase separation of geothermal fluids and a steam-heated CO 2 -rich meteoric water.