Deprotonation of Et2CHCO2H yields Et2CHCO2–. On collisional activation this ion forms CO2–˙, CH2 CH–, and MeCHCH–. In addition, elimination of H˙ and Et˙ yield Et(R)CCO2–˙(R = Et and H, respectively). The elimination of Et˙ is not a simple cleavage but occurs by loss of H˙ from a methyl group followed by loss of ethene. The carboxylate ion also rearranges to Et2CCO2H; this species decomposes to HO–, EtCCH2, and also eliminates the elements of C3H8 and CH4. All fragmentations have been studied using 2H and 13C labelling: for example it is proposed that loss of CH4 from Et2CCO2H occurs by a six-centre stepwise process in which the first step (formation of an incipient methyl anion) is rate determining. The collisional activation mass spectra of Et2CHCO2–, Me2CHCO2–, and Me3CCO2– are different, all showing characteristic decompositions. For example, all three ions eliminate methane; the mechanism is different in each case.