AbstractThe phase transition from the dilute‐good solvent (I) to the theta (I′) regimes, which does not exist in the earlier phase diagram presented by Flory and has not been detected by common methods such as intrinsic viscosity, was observed clearly by measurements of ultrasonic velocity in the MHz range. Furthermore, ultrasonic absorption measurements revealed that the thermal relaxation, due to local segmental motions, observed in phase I disappeared in phase I′. Ultrasonic methods allow us to ascertain the existence of a second‐order phase transition from phase I to phase I′ above the theta temperature in the phase diagram of polymer solutions. Copyright © 2004 Society of Chemical Industry