The polydispersity of the molar mass of a material influences the behaviour of its dynamic rheological propertiesG′ (ω) andG″ (ω). This is exemplified by the deviation from unity of the indexI = [2 − (d logG′/d logω)]/[1 − (d logG″/d logω)] in the terminal zone, i.e. at low frequencies. For a normal logarithmic distribution of species, a quantitative correlation may be established between the rheological properties and the parameter that is characteristic of the polydispersity for these species. This correlation can, in certain cases, be drawn from measurements in just the terminal zone or it may require measurements both in the terminal zone and at the beginning of the plateau zone. In each case, the parameter of dispersion can be determined by simple graphical or numerical methods. Finally, an example of the application of these methods to entangled liquid polymers is presented.