AbstractDielectric measurements were made in the frequency range from 30 Hz to 3 MHz for a series of the solutions of poly(2‐methylpent‐1‐ene sulfone) (PMPS) having two different molecular weights in benzene at 30°C. The unperturbed chain molecular dimension was estimated from dielectric relaxation time at the critical polymer contents,Ccr. This value of the unperturbed dimension was much higher than that from the intrinsic viscosity measurement. The dielectric motion atCcrinvolves simultaneous co‐operative reorientations of about three spheres. The number of the spheres of reorientation may be related to the flexibility of the molecular chain of the polymer. The ratio of two effective volumes of the polymer obtained from the unperturbed chain molecular dimension and from the dielectric relaxation time, τmagreed with that of the two effective volumes obtained from theKvalue in Onogi's treatment and from th