The non-decomposing molecular ions of methyl cyclopropanecarboxylate (14) are found to rearrange to ionised methyl but-3-enoate (15). For ions with sufficient internal energy to decompose, this isomerization is in competition with · OCH 3 loss, via direct cleavage of the ester group. Collisional activation spectroscopy may be used to distinguish between the C 3 H 5 CO + ions formed by · OCH 3 loss from the molecular ions of 14, 15 and other isomeric precursors. Four distinct C 3 H 5 CO + species (18-21) can be identified in this way; these C 3 H 5 CO + ions may themselves decompose, via CO elimination. Consideration of the metastable peak shape for CO loss, in conjunction with collisional activation spectroscopy on the resulting C 3 H 5 + -ions, leads to two main conclusions, (i) Two C 3 H 5 + ions (22 and 27) exist in potential energy wells. The very narrow metastable peaks for CO loss from 19 and 21 (leading to 22 and 27, respectively) show that these processes are continuously endothermic. In contrast, CO loss from either 18 or 20 gives rise to much broader metastable peaks. This suggests that rate-determining rearrangement of the incipient C 3 H 5 + cations, to a more stable isomer, occurs prior to decomposition, (ii) Elimination of CO from the [M- · OCH 3 ] + fragment of 14 gives rise to a composite metastable peak, thus indicating the occurrence of two competing channels for dissociation. These channels are assigned to CO loss from 18 (larger kinetic energy release) and CO loss from 19 (smaller kinetic energy release).