Acetaldehyde molecular ions were produced in an ICR cell using (2 + 1) resonance-enhanced multiphoton ionization of a pulsed molecular beam of acetaldehyde via a two-photon resonant 3s <-- n Rydberg transition in the wavelength range 364-354 nm. Mass-selected REMPI spectra of CH3CHO are presented. The power dependencies of ion intensities were examined to help establish the mechanisms of REMPI dissociation processes. The product branching ratios from REMPI dissociations of CH3CHO and CH3CDO were determined. A multireference configuration interaction method was employed to obtain structural and energetic information about the (X) over tilde (2)A’, (A) over tilde (2)A ", and (B) over tilde (2)A’ states of acetaldehyde molecular ion. Comparisons of theory with the experiments lead us to conclude that acetaldehyde molecular ions are excited to the (B) over tilde (2)A’ state and undergo both alpha and beta cleavage processes. The alpha C-C cleavage yields the formyl ion without hydrogen scrambling. The product branching ratio of [HCO+]/[CH3CO+] remains constant at 2.1 +/- 0.2 over the entire wavelength range. CH3CDO yields the branching ratio of alpha C-D to beta C-H cleavage, [CH3CO+]/[CH2DCO+] = 4.7. An aldehydic deuterium substitution reduces the branching ratio of alpha C-H(D) to alpha C-C cleavage.