The measurement of the depth of reaction of F2 into the surface of polyethylene and polypropylene is reported. The roughness of the reacted surface has required the development of a novel three-layer, coated sample and subsequent theoretical analysis and analytical techniques. The results with polypropylene can be confirmed by visible reflectance fringes; however, in polyethylene the three-layer method is, at present, the only easy way to measure the extent of fluorine modification of the polymer surface. The extent of fluorination was found to be proportional to the time of exposure to F2 gas and to increase with elevation of the sample temperature.