A process for preparing fluoridated surfaces of polymers includes exposing the surfaces for a period of time to the influence of a fluorine containing a treatment gas of defininte proportion for forming fluoridated hydrocarbons. A small portion of the treatment gas is drawn off during the exposure period and channeled via an activated aluminum oxide contact for carrying out the reaction Al.sub.2 O.sub.3 +3F.sub.2 .fwdarw.2AlF.sub.3 +3/20.sub.2 to determine the fluorine concentration in the residual gas by means of quantitative determination of the released oxygen and therewith the degree of fluorine coating of the surface.