A transition energy scale was established on the basis of di-Me indoaniline as org. dye for 23 mono- and bidentate polar additives. From polymn. expts. of isoprene with s-butyllithium in mixts. of n-hexane and the selected bases, the microstructure of polyisoprenes was detd. by 1H-NMR spectroscopy. The vinyl contents of these polymers could be satisfactorily correlated with the transition energy measurements. However, it turned out that esp. bidentate bases do not fit into this scheme. Obviously, besides their influence on the polarity of the reaction soln., they exhibit special interactions with the growing centers. The developed correlations allow the prediction of 1,2- and 3,4-linkages in polyisoprenes from anionic polymn. of isoprene with butyllithium in polar solvents in the presence of low concns., also of bases hitherto not investigated. The temp. dependence of the microstructure can likewise be estd. As in the case of butadiene, at least for weak bases, it was found that the vinyl contents depend primarily on the polarity of the reaction mixt. detd. by the base concn. and not on the molar ratio of base to initiator.