A study of the effect of acetaldehyde, trans-butene-2, and isobutane pressure on the fluorescence and phosphorescence emissions from acetaldehyde vapour has been made. The results have been interpreted in terms of electronically excited singlet and triplet states, in the behaviour of which vibrational energy content plays a crucial role. The extent to which the initially formed excited state undergoes predissociation, or is collisionally deactivated, has been examined from 254 to 340 nm between 20 and 99 degrees C. At the longer excitation wavelengths the effect of pressure on emission yields indicates that collisional deactivation to the vibrationally equilibrated levels of the excited singlet state followed by intersystem crossing to the triplet state is the predominant mode for energy dissipation. At shorter excitation wavelengths predissociation becomes of increasing importance. Collisional deactivation of excited singlet acetaldehyde molecules by acetaldehyde and trans-butene-2 occurs by a single stage mechanism, in contrast to isobutane, and the possibility of complex formation is discussed. Relative rates for collisional deactivation by butene-2 and acetaldehyde of vibrationally excited acetaldehyde molecules in the first excited singlet state have been determined and shown to vary as a function of excitation wavelength.