The dynamic mechanical thermal properties of a family of poly(α-alkyl β,L-aspartate)s bearing various cyclic, linear, and branched alkoxycarbonyl groups in the side chain were studied. The measurements carried out by dynamic mechanical thermal analysis (DMTA) revealed the significant influence of the constitution of the side chain on mechanical relaxation phenomena. Three relaxations were observed, which are referred to as γ, β, and α, in increasing order of temperature. The first two, γ and β, are related to the local and global motions of the side chain, respectively. Relaxation α is related to the motion of the main chain. Relaxation β, which is associated with the rotation of the side chain, is the most intense. The magnitude and temperature at which this relaxation occurs depends on the volume, the length, and the degree of branching of the ester group of the side chain. A comparison between the dynamic mechanical properties of poly(β,L-aspartate)s and poly(α,L-glutamate)s revealed that the two methylene groups spacing the ester group from the main chain provides the poly(α-L-glutamate)s with greater mobility, and thus, relaxations α and β occur at lower temperatures. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 994–1003, 2006