On the basis of so-called linear free enthalpy correlations, a method is developed to predict vinyl contents of polybutadienes from anionic polymerizations with butyllithium in nonpolar solvents in the presence of bases. According to Reichardt with an extension by Langhals, a transition energy scale for the solvatochromic dye N,N-dimethyl indoaniline in dependence of concentrations of some bases is established. The bases were taken from polymerizations of butadiene of Antkowiak et al. and of Jin et al. It could be shown that vinyl contents determined by these authors correlate well with measured transition energies over the whole concentration range of the bases. Also, temperature dependences of vinyl contents can be predicted satisfactorily. Furthermore, it was found that vinyl contents in polybutadienes depend for weak bases primarily on the polarity of the solution mixture determined by the base concentration and not on the molar ratio of base to initiator. The results are extensively discussed in comparison with earlier attempts in the literature to predict the microstructure in polybutadienes in dependence of polar modifiers.