The behavior of mineral oils under high hydrostatic pressures is studied. It is found that a test oil converts from a liquid to a solid faster than within 9 μs when the time mean value of the pressure is 1,200 N/mm2(— 12,000 atmospheres). As the solidification velocity for liquids increases approximately proportional to the pressure rise above the static solidification pressure, this solidification time shows that rolling element bearings under normal working conditions have solidified oil in the high pressure part of the contact.