AbstractAn equation describing the relationship between the GPC determined breadth of the molecular weight distribution, (σn/Mn)G, the true breadth of the distribution,σn/Mn, zone spreading or tailing ΔVe, and the separability employed,K(M), has been derived:.HereK(M) is the absolute value of the derivativedVe/dM, in whichVeis elution volume andMis molecular weight. The validity of this equation was established through GPC fractionations of blends of narrow molecular weight distribution polystyrenes. Application of the equation to experimental data shows that ΔVeincreases with increasingM. The equation indicates that GPC molecular weight data are less in error with polymers of broader molecular weight distributions and with larger values ofK(M). The importance of the experimental variableK(M) is that it defines the degree of fractionation, higher values ofK(M) denoting more complete fractionat