The mean hydrodynamic radius of poly(p-isopropyl α-methylsty-rene) in toluene has been determined at 21.5° C as a function of weight-average molecular weight Mwfrom 18,000 to 625,000 dalton. The measurements of hydrodynamic radii were made using photon correlation spectroscopy with a convenient specially designed filtering system to remove dust from the samples. The coefficients in the relationship D=CM−ahave been determined for our polydisperse samples. Although the polydispersity severly affects the determination of C, we have determined a=0.57 ± 0.03 in agreement with theories for linear chains in good solvent. We also report the first determinations of the Huggins coefficients for poly(p-isopropyl α-methylstyrene).