A dansyl labeled polyelectrolyte (ADDan60) was prepared by copolymerization of ionizable monomer 2-(acrylamido)-2-methylpropanesulphonic acid (AMPS) with N,N-dimethylacrylamide (DMAA) and N-[2-[[[5-(N,N-dimethylamino)-1-naphthalenyl]-sulfonyl]amino]ethyl]-2-propenamide (DANSAEP, 0.2 mol%). Reduced viscosity eta(sp)/c, electrolytic conductivity kappa, and fluorescent spectrum of ADDan60 in DMSO/THF mixtures were measured as functions of the volume fraction V-THF Of THF. In the region of V-THF < 0.7, the emission maximum lambda(em), relative intensity R-F of ADDan60 in DMSO/THF to that in DMSO, and anisotropy ratio r indicated that the polymer maintained its solvation layer not to change so much as the bulk solvent and exhibited a polyelectrolyte behavior, as observed from eta(sp)/c and kappa. Once V-THF was beyond 0.70, the solvation layer was changed leading to the ion pair formation in an avalanche manner, which induced chain aggregation and phase transition due to the dipole attraction. The results suggest a behavior transition in ionizable polymers from polyelectrolyte into ionomer induced by medium polarity.