Neutrals CCCO, CC¹³CO, CCCS and CC¹³CS have been prepared by one-electron vertical (Franck–Condon) oxidation of the precursor anion radicals (CCCO)⁻˙, (CC¹³CO)⁻˙, (CCCS)⁻˙ and (CC¹³CS)⁻˙ respectively in collision cells of a reverse sector mass spectrometer. Ionisation of the neutrals to decomposing cations shows the neutrals to be stable for the microsecond duration of the neutralisation–ionisation (⁻NR⁺) experiment. No rearrangement of the label in energised CC¹³CO or CC¹³CS occurs during these experiments. In contrast, minor rearrangement of (CC¹³CO)⁺˙ is observed [(CC¹³CO)⁺˙ → (OCC¹³C)⁺˙], while significant rearrangement occurs for (CC¹³CS)⁺˙ [(CC¹³CS)⁺˙ → (SCC¹³C)⁺˙]. Theoretical calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory show that the cationic rearrangements occur by stepwise processes via key rhombic structures. Overall, the degenerate processes result in O and S migration from C-3 to C-1. The cations (CCCO)⁺˙ and (CCCS)⁺˙ require excess energies of ≥ 516 and ≥ 226 kJ mol⁻¹ respectively to effect rearrangement.