The peak broadening in size exclusion chromatography of seven commercial polystyrene (PS) standards with narrow molar mass distribution (MMD) and of 6-9 polypropylene (PP) fractions (MBAR(w)/MBAR(n) = 1.34-2.10) obtained by direct extraction was determined in four different column sets. The dependence of the Gaussian peak broadening parameter, sigma2, on the peak elution volume was different for PS and PP. For a commercial grade of controlled rheology PP (CR-PP) (Daplen PT55) with MBAR(w)/MBAR(n) < 3 it was shown that disregard of the peak broadening effect leads to considerable errors in the MMD at low and high molar mass and in the polydispersity parameter MBAR(w)/MBAR(n). The same lot was studied in an Austrian National Research program for its physical properties in processing and application and in a round-robin test of IUPAC working party IV.2.2. on molecular characterization of commercial polymers.