Dielectric absorption studies have been made of a1dehyde group relaxation of aromatic aldehydes in a polystyrene matrix. For aldehyde group relaxation in terephtha1dehyde, the Eyring parameters are 6H -1 E = 29.3 kJ mol and 6S -1 -1 E = 15.1 JK mol • We found that changing the medium from polystyrene to polypropylene or polyethylene has little effect on the activation parameters for the intramolecular process whilst causing a considerable change for the molecular process, the activation parameters forming a sequence based on the efficiency of packing of the polymer chain. While examining steric effects, we found that the aldehyde group relaxes faster in the o-halo-benzaldehydesthan in terephtha~dehyde, possibly due to a limited angle reorientation of the aldehyde group. The 3-substituted aldehydes show a tremendous unexplained increase in the bHE over the usual group value,6HE ~al1ing from fluorine to iodine in the m-halo-benzaldehydeso The influence of substituents with the greatest possible electron donating or accepting powers has also been examined. For p-dimethylaminobenzaldehyde and other highly conjugated ~olecules, the activation enthalpy is found to be much larger than the usual group value. In some molecules, the presence of a symmetry in the activated state seems to lower 6SE, while in others, this effect has been outweighed by other factors. A theoretical approach to activation entropies starting from partition functions shows that the internal rotational partition function does not contribute to the activation entropy for internal rotation, the contribution coming from other factors such as vibrationo Barrier heights have been calculated by CNDO/2 and INDO molecular orbital methods and are underestimatedo Quantum tunnelling is found to be important in the study of activation entropies. Our ~HE results for the p-halo-benzaldehydes differ appreciably from the V2 values reported for the gaseous phase from the far-infrared approach. In general, our results are in fair agreement with n.m.r. work.