The bottom outflow from the Persian (Arabian) Gulf, which intrudes in the Gulf of Oman as an intermediate salinity maximum and spreads in the northern Arabian Sea, was inferred to be seasonal. Also, based on particular expedition, single values of temperatures, salinities, and oxygen concentrations were assumed as end members for this core layer. From historical data it is shown here that the salinity of the water exiting at depth from the Gulf is first reduced to <40 ppt by mixing with Gulf of Oman water in the shallow Strait of Hormuz with its strong tides. The core layer acquires its characteristics when the mixing product descends the outer shelf at the head of the Gulf of Oman and entrains upper-thermocline water with an intermediate to low oxygen content above the outflow. After leaving the seabed, when mixing with water also below the intrusion begins, the maximal values observed in the core layer for salinity, density (sigma-t), and oxygen saturation ranged between 37.5 and 38.0 ppt, 26.30 and 26.95 g dm−3, and approximately 20 and 60%, respectively (eight expeditions, with two for oxygen; in all, 15 temporally separated cruises). Rules about this variability could not be recognized, except that the oxygen content in the freshly formed core layer seems to be highest in spring and lowest in fall. At the head of the Gulf of Oman, one intrusion was always present; at least three times, two intrusions of nearly the same density were encountered, as is common also several 100 km away from the source region. Persistent presence of the core layer at the head, but absence near the mouth of the Gulf of Oman in the spring of one year, and presence during the following spring (five cruises between May 1975 and August 1976) do not indicate a marked seasonal pattern of the outflow. Within at least 1,000 km from the Strait of Hormuz, advection clearly participates in the lateral spreading of the core layer.