Abstract: To load oil on tankers in which the tank holds are previously filled with water, for ballast, and to avoid the formation of explosive vapors, oil is introduced at a higher pressure level into the tank hold than the pressure required to displace the water, while, simultaneously, the water is drawn off, by pumps or syphon action; preferably, the oil is introduced into the tanker from a level above that of the highest point of the tanker level, and water is drawn out from the bottom of the tank hold to a storage reservoir having its highest point below the lowest of the tank holds of the tanker. To unload the oil, oil is withdrawn from the top of the tanker hold into a receptacle below the level of the bottom of the tanker hold, and replaced by water introduced into the bottom of the tanker hold from a level above that of the tanker hold, withdrawal and introduction of fluids into the tanker hold being carried out simultaneously.