C-13 NMR spectroscopy was used to investigate the physical basis of the rheological thermal transition (T(rheo)) in symmetric poly(styrene-b-isoprene) (SI) diblock copolymers. Among the studied samples, those with M(n) = (3 - 32) x 10(3), which all exhibited two glass transition features (T(g,I) and T(g,S)) in their DSC curves, had T(rheo) values ranging from -10-degrees-C to 205-degrees-C. The NMR line width data related to the styrene units in the bulk materials show that a strong segregation behavior persists over a wide range of temperature above T(rheo). For M(n) > 1.2 X 10(4), the rheological transition coincides with the last trace of ordering (extinction of the lamellar grain diffraction), but in the disordered state the materials contain polystyrene (PS) block heterogeneities large enough to exhibit NMR features comparable to those of homo-PS samples having the same T(g) as T(g,S). Over this range, T(rheo) increases in the same ratio as M(n)0.35. An analysis of this relationship suggests an order-disorder transition governed by an energy barrier related to the interface tension in the ordered state. For Mn < 1.2 x 10(4), T(rheo) no longer corresponds to an order-disorder transition because vitrification of the PS block heterogeneities occurs before a lamellar ordering can take place in the materials.