Publisher Summary The chapter describes procedures for the preparation and isolation of yeast tRNA Phe carrying Prf or Etd in place of YWye or dihydrouracil. For the separation of both tRNA Phe YWye 37 and tRNA Phe –Etd37 from unreacted tRNA -Phe YWye , reversed phase chromatography (RPC-5) proved to be advantageous over the previously employed BD-cellulose. A similar, although less well resolved, elution profile was obtained with tRNA Phe Prf35 . After RPC-5 chromatography, the phenylalanine acceptance of the tRNA Phe derivatives was sometimes found to be rather low, depending on the isolation procedure. The best results were obtained when the tRNA was precipitated with ethanol from the eluate and washed by two additional precipitations. The principle of procedure C is also used for the determination of the condensation products of primary amines with tRNA Phe YWye . However, in this case the condensation product is also reduced by [ 3 H]NaBH 4 . Therefore, the 3 H-labeled C-1 of the ribitol in reduced and tRNA_ Phe YW ye is removed by treatment with periodate.