Chain conformational distribution of a biodegradable polyester elastomer, poly(beta-hydroxyoctanoate) (PHO), has been analyzed using vibrational spectroscopy in conjunction with normal vibrational analysis. For the as-prepared, mainly amorphous materials, side-chain conformation distribution is extremely broad and is consistent with the expected three-state rotational isomeric model. On the basis of our vibrational analysis, we have established that side-chain conformational distribution transforms to one containing mainly the extended form after thermal annealing. In contrast, the conformational distribution of the side chains remains unchanged as a function of strain. Additionally, the carbonyl stretching vibrations were found to be sensitive to changes in the crystalline environment and thus can be utilized to monitor the degree of crystallinity in these samples. Even though the exact mechanism has not been identified, these results indicate that changes in crystallinity or side-chain conformational distribution cannot be responsible for the extremely high tensile set observed.