Various .sup.13C and .sup.14C carbon labeled compounds R.sub.f--.sup.bCF(U).sub.y(T).sub.zFormula 1A or R.sub.f--.sup.bCF.sub.2.sup.bCF(U).sub.y(T).sub.zFormula 1B wherein R.sub.f is a linear or branched perfluoroalkyl radical Z(C.sub.mF.sub.2m)--, m is 1, 3 to 20, or a mixture thereof; Z is F when m is 1, and Z is F, Cl, or H when m is 3 to 20 or a mixture thereof; b is 13 or 14; y is 1, and z is 0 or 1; and when y and z are each 1, U is F, and T is selected from the group consisting of monovalent radicals --X wherein X is I or Br, --CH.sub.2--CH.sub.2--X, --CH.dbd.CH.sub.2, --CH.sub.2--CH.sub.2--OH, --CH.sub.2--COOH, --CH.sub.2OH, --.sup.bCH.sub.2OH, --COOH, --.sup.bCOOH, --O--CF(CF.sub.3)--C(O)F, --O--CF(CF.sub.3)--X, --SO.sub.2Y wherein Y is H, F or Cl, and --SO.sub.3H; and when y is 1, and z is 0, U is selected from the group consisting of divalent radicals .dbd.CH--COOH, .dbd.CH--CH.sub.2--OH, and .dbd.O, are disclosed, and a process for their preparation which selectively introduces .sup.13C and .sup.14C carbon labels into a fluoroalkyl chain.