The difference of the relation of molecular weight (MW) and retention volume of narrow polystyrene (PS) samples from broad ones in the size exclusion chromatography - light scattering (LS)/refractive index (RI) detectors is discussed. Effects of interdetector delay volume on the relation was serious and the approach to adjust the interdetector delay volume so as to get a plateau relation of MW and retention volume for PS samples with a narrow MW distribution was proposed. The interdetector delay volume was found to be a function of MW. PS samples with a broad MW distribution had the similar relation of MW and retention volume to the calibration curve constructed with PS standards with narrow MW distributions; and the MW averages obtained by the LS/RI approach gave the comparable values to those calculated by a calibration curve method.