REMPI spectra are reported for the acetaldehyde (A) over-tilde (1)A", (B) over-tilde, (C) over tilde, and (D) over-tilde states. Photoelectron spectroscopy is used to probe the nature of the intermediate states, measure the cation vibrational frequencies, and to identify useful routes for preparing state-selected ions. Ab initio calculations of neutral and cation vibrational frequencies are also reported. The (B) over tilde state is found to be a well-behaved Rydberg state, but with some distortion relative to the cation geometry along the nu (10) and nu (15) coordinates. There are (B) over tilde state REMPI transitions that produce well state-selected cations, with vibrational energies of up to 0.4 eV, and several new cation frequencies are observed. The (A) over tilde (1)A" state gives structured, if somewhat broadened, REMPI transitions, but ionizes to produce a broad population of vibrationally hot ions. Only the origin band of the (C) over tilde state is observed in REMPI, despite high intensity for this state in absorption. A few (D) over tilde state transitions are sharp, and ionize to produce cold cations, as expected for a good Rydberg state. Most (D) over tilde state levels are strongly mixed and broadened, however, and ionize to hot cations. Inconsistencies in the literature are discussed in light of the photoelectron spectra and ab initio results. (C) 2001 American Institute of Physics.