The reaction between O⁻˙ and MeO–CH₂–CC–CDO in the ion source of a VG ZAB 2HF mass spectrometer gives a number of product anions including [H₂CCCCO]⁻˙ and [HCCCCDO]⁻˙ (in the ratio 1 : 5). Neutralisation–reionisation (⁻NR⁺) of [H₂CCCCO]⁻˙ results in the sequential two-electron vertical oxidation [H₂CCCCO]⁻˙ → H₂CCCCO → [H₂CCCCO]⁺˙. Singlet H₂CCCCO lies 158 kJ mol⁻¹ below the triplet [at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory]. The majority of neutrals H₂CCCCO are stable for the microsecond duration of the NR experiment, but some are energised and decompose to give H₂CCC and CO. A similar ⁻NR⁺ experiment with [HCCCCDO]⁻˙ yields neutrals HCCCCDO, some of which are excited and rearrange. Calculations show that it is the singlet form of HCCCCHO which rearranges (the singlet lies 36 kJ mol⁻¹ above the ground state triplet): the rearrangement occurs by the sequencial H transfer process, HCCCCHO → HCC(CH)CO → H₂CCCCO. Neutral HCCCCHO needs an excess energy of only 43 kJ mol⁻¹ to effect this reaction, which is exothermic by 230 kJ mol⁻¹. Both HCC(CH)CO and H₂CCCCO formed in this way should have sufficient excess energy to cause some loss of CO. The anions [CC(CH)CHO]⁻˙ and [CC(CD)CHO]⁻˙ are formed in the ion source of the mass spectrometer by the reactions of HO⁻ with Me₃SiCC–CHCHOMe and Me₃SiCC–CDCHOMe respectively. ⁻NR⁺ of these anions indicate that energised forms of CC(CH)CHO and CC(CD)CHO may rearrange to isomer(s) which decompose by loss of CO. Singlet CC(CH)CHO rearranges to HCC(CH)CO and H₂CCCCO, both of which are energised and fragment by loss of CO.