Turbulent drag reduction (DR) in a rotating disk apparatus was examined for a homologous series of polyisobutylene (PIB) with two different solvent systems to investigate the effect of solvent on universal DR characteristics. The concentration dependence of DR for these systems obeys an empirical universal DR equation. A linear correlation between polymer concentration (C) and C/DR for different molecular weights of PIE was also obtained. The intrinsic concentration was found to be an extremely useful quantity in normalizing the DR data for a homologous series of PIE. The characteristic value, K, was found to depend upon the solvent system.