Molecular weight dependency of a specific UV absorption coefficient of polystyrene in solution was determined by supercritical fluid chromatography applied to equimolar and equimass mixtures of uniform polystyrenes whose degree of polymerization was n = 1 to 40. From the ratio between the response from a UV detector and that from a refractometer, we determined the refractive index increment dn/dc as a function of molecular weight for polystyrene in tetrahydrofuran at the wavelength gimel = 633 nm. We found that dn/dc is well expressed as a linear function of the reciprocal of molecular weight for uniform polystyrenes. (C) 2004 Wiley Periodicals, Inc.