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We report on the detailed mass spectrometric analysis of the degradation products generated during storage of poly(methyl methacrylate) (pMMA) and polystyrene (pSty) carrying cumyldithiobenzoate (CDB) endgroups. Samples were stored in either a cyclic ether (tetrahydrofuran) (THF) or an inert solvent (dichloromethane). The degradation process was followed over a period of 4-weeks. Degradation rate of the reversible addition fragmentation (RAFT) polymer strongly depends on the hydroperoxide-content of the solvent. Mass spectrometric evidence supports an unexpected radical degradation mechanism for the pMMA macroRAFT agent. Hydroperoxide functional pMMA was the single product after less than 7 days in high purity THF. No formation of the sulfine/thioester was observed. The identity of the hydroperoxide was unambiguously assigned using accurate mass measurements by Fourier-Transform ion-cyclotron-resonance mass spectrometry together with chemical identification reactions. The hydroperoxide end group formation proceeds efficiently as well as in high yields and thus constitutes a powerful method for end group modification. The degradation pathways of the CDB functional pSty in THF include mainly oxidation towards the sulfine/thioester, with little degradation via thermal elimination of dithiobenzoic acid and subsequent epoxidation. The shelf life of CDB functional polymers is limited even in inert solvent because of this inherent but slow thermal elimination reaction. Because of the short period necessary for the transformation of the functional dithiobenzyl endgroups, substitution of cyclic ethers as solvents for RAFT polymers in synthesis and analysis is strongly suggested. © 2008 Wiley Periodicals, Inc.