Abstract: A process for liquefying gaseous substances is disclosed wherein a first fluid to be liquefied is conveyed along a first path including a portion in which it is cooled. A cooling fluid is conveyed along a second path including a first section in which it is cooled, a second section in which it exchanges heat with the first fluid so as to cool the latter, and a third section thermally separated from the aforementioned first section of the second path. A precooling fluid is conveyed along a third path including a first part also thermally separated from the aforementioned first section of the second path and wherein it exchanges heat with the cooling fluid in the third section of the second path so as to be cooled thereby. The third path further includes a second part wherein the precooling fluid exchanges heat with the cooling fluid in the first section of the second path so as to cool the same.