Abstract: An electric immersion heating apparatus for heating highly corrosive liquids includes a flexible cable-type immersion heater having a continuous coiled resistance heating element disposed in axially spaced pitches and being enclosed in a continuous tubular outer jacket formed of a flexible plastic material, such as polytetrafluoroethylene with a PFA side chain, resistant to chemical attach by highly corrosive liquids. The outer jacket is received over the coiled heating element in closely fitting relationship and has a wall thickness sufficiently thin to provide efficient heat transfer from the coiled element to the liquid being heated. Because of its thinness the outer tubular jacket is subject to permeation of damaging corrosive vapors through the jacket to the coiled heating element.