Medium-resolution laser induced fluorescence excitation spectra of jet-cooled acetaldehyde and fury deuterated acetaldehyde have been recorded using a pulsed molecular beam apparatus. Analysis of the rotationally resolved S-1 double left arrow S-o origin yields quantitative information regarding the structure for the S-1 excited state of the molecule. Ab initio, self-consistent-field calculations have been performed as a check on the uniqueness of the fit obtained to the experimental data. In accord with previous predictions, the electronic excitation is found to be an pi(*) double left arrow n c-type(perpendicular to) transition. This results in a loss of planarity in the CCOH group, and an extension of the CO bond length relative to the ground-state form. The position of the methyl hydrogens also changes from an eclipsed to a staggered conformation with the CO bond. The excited-state rotational constants for the vibrationless level are found to be A’= 1.596 +/- 0.007, B’ = 0.334 +/- 0.005, and C’ = 0.305 +/- 0.006 for the CH3CHO isotope and A’ = 1.036 +/- 0.007, B’ = 0.286 +/- 0.006, and C’ = 0.253 +/- 0.006 for the CD3CDO isotope.