Abstract 1H T 1 relaxation times were measured in solutions of poly (γ-benzyl- l-glutamate), PBLG, and poly(β-benzyl- l-aspartate), PBLA, and 13C T 1 relaxation times, NOE factors and line widths in solutions of the two polypeptides and of poly( l-alanine), PLA, over a broad range of solvent composition (CDCI 3—trifluoroacetic acid) including the helix—coil transition. PBLG was also studied through temperature-induced inverted helix—coil transition. Most relaxation parameters change smoothly over the studied range, and the observed changes correspond to increase of mobility with increasing content of trifluoroacetic acid and with increasing temperature except for PBLG which undergoes an inverted helix—coil transition with temperature. Analysis of experimental data by means of the isotropic model yielded, for the backbone carbons of all three polypeptides, τ eff ∼ 2 × 10 −8 sec in the helix and τ eff ∼ 1 × 10 −9 sec in the coil form. For the side-chain carbons, τ eff decreases in the range 10 −9 − 10 −10 sec with increasing distance from the backbone, with τ eff values consistently lower in the coil form. Results of analysis by means of a model with two correlation times are discussed.