The quantitative evaluation of the concentration of the propagating species in the polymerization of butadiene initiated by butyllithium in cyclohexane in the presence of complexing agents such as 1,2-dipiperidinoethane (DIPIP) and diglyme is described. The method, which may be applied to polymerizations other than the one reported here, is based on the precise knowledge of the molar extinction coefficients of the polyanion in the absence of complexing agent and the measurement of the absorbance at two different wavelengths when complexing agent is present. These measurements are made in the region 250-400 nm. The method has the advantage that it is not necessary to have any independent measure of the amounts of initiator and complexing agents used at the start of reaction. From the u.v.-visible spectroscopic studies, a slow post-polymerization reaction between polybutadienylithium and DIPIP is noted. Molecular weight and i.r. measurements on the polymers produced in the presence of DIPIP support this suggestion and a mechanism is proposed for this reaction.