The viscosity of copper ethylenediamine (CED) solutions of pulps exhibits a decrease with time, while the viscosity loss is insignificant when the time between dissolution and measurement is just 15-20 min. Time of 1 h or longer would lead to noticeable differences from the initial viscosity. The viscosity of the solution at the beginning can be estimated from a series of measurements performed under strict conditions. In particular, tight control of the temperature of the solution and accurate measurement of the flow time are of importance. The plot of efflux times against elapsed times to the measurements results in a almost linear correlation, which enables to determine the initial viscosity.