AbstractA relation between the limiting viscosity number [η] and the weight average molecular weight of polyethyleneimine (PEI) is proposed. Special care was taken to suppress the marked polyelectrolytic character of PEI in water for both viscometric and light‐scattering measurements. For these measurements various fractions were isolated in three different ways from some commercially available PEI′s, all having broad molecular weight distributions. To obtain an impression of the molecular weight distribution of the resulting fractions, some osmotic measurements to obtain the number average molecular weight and some ultra‐centrifuge runs were per