A method is developed whereby the autocorrelation spectra of light scattered inelastically from dilute polymer solutions can be used to determine polymer number-averaged and weight-averaged molecular weights. The method is based on transforming the polymer z-averaged diffusion coefficient and its relative dispersion, extracted from a cumulants analysis of the spectra, into number-averaged and weight-averaged molecular weights. To do this, experimentally determined constants from diffusion coefficient versus molecular weight studies are employed. The range of validity in the resulting transform expressions is subject to certain restrictions in the width and in the magnitudes of higher moments of the polymer molecular weight distribution. The expressions derived have particular potential usefulness in the determination of Mn and Mw for rodlike polymers. Mn and Mw values determined using these expressions to analyze narrow and intermediate molecular weight polystyrene samples are in good agreement with both the manufacturers data and gel permeation chromatographic analyses. © 1979, American Chemical Society. All rights reserved.