Fluorescence decay of the S-1 state of d(4)-acetaldehyde is measured. Below the dissociation threshold for formation of fragments of formyl and methyl radicals, single exponential decays of fluorescence are observed. Biexponential decay is observed when the excitation energies are near and above the dissociation threshold. Hence, in this region the mechanism for intramolecular relaxation of energy reaches the "intermediate case." Strong coupling between S-1 and T-1 states or reversible reaction for S-1 <-> T-1 results in this biexponential behavior in the fluorescence decay of S-1. Rates of appearance of DCO from dissociation of acetaldehyde are measured. The stepwise increases in plots of both rate of appearance of formyl radicals and rate of decay of excited acetaldehyde versus excitation energy for both isotopic variants of acetaldehyde are observed. According to fits to Rice-Ramsperger-Kassel-Marcus (RRKM) theory, these sharp increases correspond to vibrational levels of transition state in the triplet surface. Frequencies of torsional and C-C-O bending modes are determined to be (73 +/- 10) and (200 +/- 10) cm(-1) for the transition state of CD3CDO and the torsional frequency (80 +/- 10) cm(-1) for CH3CHO. That these fitted frequencies are less than values calculated with methods B3LYP and MP2 indicates a looser transition state than calculated for dissociation. From these fits the dissociation threshold for formation of radical fragments is determined to be 31 845 for CD3CDO and 31 650 cm(-1) for CH3CHO.