Torsional braid analysis (TBA) (∼0.3 Hz) and differential thermal analysis (DTA) data are presented for the temperature for the region 0–200°C for two series of atactic polystyrenes with narrow molecular weight distributions: (a) anionic series, M̄n = 600–2×106, M̄w/M̄n ≃ 1.1; (b) fractionated thermal series, M̄n = 2,000–1.1×105, M̄w/M̄n < 1.25. Preliminary results on bimodal blends are also reported. Heating and cooling cycles were employed with TBA; only the heating mode was used with DTA. In addition to a dynamic mechanical loss peak at Tg, a higher temperature loss peak was also found. Designated the Tll or liquid–liquid transition (relaxation), its temperature is 1.1 to 1.2 Tg (°K) for polymers with molecular weight below the critical molecular weight (Mc) for chain entanglements. Above Mc ≃ 35,000, it rises steeply, being ≃200°C for M̄n = 110,000. The common dependence of Tg and Tll on M̄n−1 below Mc suggests a common molecular origin. The two facts, (a) that Tll > Tg and (b) that Tll reflects chain entanglements, further suggest that Tll involves a longer chain segment length and possibly the entire molecule. Comparison of Tll versus log M plots with T versus log M isoviscous state plots based on zero-shear melt viscosity data from the literature implies that Tll measured by the TBA technique corresponds to an isoviscous state of 104–105 poises. The employment of narrow molecular weight polymers is presumably responsible for both the linear variation of the Tll transition with M̄n−1 (which suggests a free volume basis for the relaxation) and the form of the variation of the Tll transition with log M (which suggests an isoviscous basis for the relaxation). The sharpness of the Tll loss peak by TBA decreases with increasing molecular weight and dispersity. The DTA endothermic event corresponding to Tll is clearly related to the occurrence of flow since the fused films which result from heating granules to 200°C and cooling to R.T. do not reveal a Tll on reheating. If a fused film is crushed, a Tll event is observed on heating. For bimodal blends with M̄n < Mc for both components, the Tll transition was averaged; with one component less than and one greater than Mc, the Tll transitions of the components appeared to occur independently at temperatures corresponding to those of the isolated components. In accordance with Ueberreiter and Orthmann, Tg appears to separate a glassy state from a fixed liquid state, whereas Tll separates the fixed liquid from a true liquid state. Possible molecular interpretations for the Tll process are discussed. Systematic bodies of data from the literature which indicate the presence of the Tll process in other polymers are summarized.