Abstract: In order to produce a preform or parison from which an optical fiber can be drawn, a vitrifiable substance such as silicon chloride and a doping agent such as germanium chloride, for example, are fed in liquid form and at room temperature to a spray nozzle together with water or steam for continuous deposition on a relatively rotating and axially reciprocating inner or outer surface of a cylindrical support until that surface is covered with a heavy layer of as yet unvitrified hydrated silica admixed with the doping agent in proportions designed to provide a selected refractive-index profile. The resulting preform, after degassing and separation from the support if necessary, is freed from occluded water molecules by exposure to a flow of hot dehydrating gas which may contain anhydrous compounds and is thereafter collapsed into a solid rod and completely vitrified at high temperatures.