The nuclear magnetic resonance (NMR) relaxation has been studied in polyisobutylene and polybutadiene at temperatures from −170° to +200°C and at three resonance frequencies: 20, 30, and 50 Mc/sec. In polyisobutylene, the spin‐lattice relaxation time (T1) passes through two minima with change in temperature. The low‐temperature minimum is ascribed, as in other compounds, to methyl‐group rotation, but in polyisobutylene this motion is found to encounter relatively large hindrance, presumably owing to inter‐locking among the groups. The high‐temperatureT1‐minimum is ascribed to rotational and translational motion of the segments. The extent of motion is qualitatively gauged by calculation from the Bloembergen‐Purcell‐Pound theory for NMR relaxation in simple liquids.T1is insensitive to molecular weight (M.W.) over a wide range, but the dependence ofT2upon M.W. changes abruptly when M.W.≅4×104. In polybutadiene,T1is found to depend markedly on thecis‐transcontent.