The Invention concerns an apparatus for and method for the simultaneous determination of permeation rate through and desorption rate from a polymeric specimen, the method comprising sealingly separating a first volume from a second volume with a specimen to be tested, said specimen being suffused with a first isotope of a gas or vapor; introducing in said first volume a second isotope of said gas or vapor, said second isotope being detectably distinguishable from said first isotope; in said second volume, adjusting the partial pressure of both said isotopes of said gas or vapor to a negligible value compared to that of said second isotope of said gas or vapor in said first volume; and, distinguishably detecting the concentration of each said diffusing isotope of said permeant in said second volume.