The nature and circulation of water masses in the Persian/Arabian Gulf (hereinafter referred to as the Gulf) is investigated by examination of a historic database of hydrographic observations. The densest water forms in winter at the northern end of the Gulf rather than along the warmer southern and western coasts. With the exception of small amounts of water directly above the seafloor, most water flowing out of the Gulf mixes across a density front that separates Gulf Deep Water within the Gulf from the Indian Ocean Surface Water (IOSW). Contrary to previous inferences, the seasonally variable incursion of IOSW into the Gulf peaks in late spring. This timing may be due to seasonal changes in sea surface slope driven by variations in evaporation rate. In order to explain mooring results published elsewhere that show relatively small seasonal changes in the volume flux through the Strait of Hormuz (hereinafter referred to as the Strait), we suggest that this flux is driven by the difference between the density of Gulf Deep Water in the interior of the basin and water at comparable depths outside the Gulf. This density difference varies less than 15% during the year. High rates of vertical mixing in the Strait extend about 200 km westward in response to topographic constriction of tidal flows by islands and shoals.