Abstract Photoionization and dissociative photoionization of acetaldehyde (CH 3 CHO) in the 10.0–13.7 eV energy range are studied by using synchrotron radiation double imaging photoelectron photoion coincidence spectroscopy (i 2 PEPICO). The X 2 A′ and A 2 A“ electronic states of CH 3 CHO + as well as the Franck‐Condon gap region between these two states have been populated with several vibrational sequences and assigned in the high‐resolution slow photoelectron spectrum (SPES). The adiabatic ionization energies (AIEs) of the X 2 A′ and A 2 A” states are measured at 10.228±0.006 and 12.52±0.05 eV, respectively. The present results show that the X 2 A′ state is a stable state while the A 2 A“ state is fully dissociative to produce CH 3 CO + , CHO + and CH 4 + fragment ions. The 0 K appearance energies ( AE 0K ) of CH 3 CO + and CHO + fragment ions are determined through the modeling of the breakdown diagram, i. e ., AE 0K (CH 3 CO + )=10.89±0.01 eV (including a reverse barrier of ~0.19 eV) and AE 0K (CHO + )=11.54±0.05 eV. In addition, the dissociation mechanisms of CH 3 CHO + including statistical dissociation, direct bond breaking and isomerization are discussed with the support of the calculated dissociation limits and transition state energies.