Protonation of polyaniline base with lightly sulfonated polystyrene in polar solvents such as dimethyl sulfoxide (DMSO) and N-methyl pyrrolidone (NMP) was investigated by UV-Vis absorption spectroscopy. The isosbestic point clearly shows that quinoid unit and semiquinoid unit are in equilibrium and is a function of the sulfonic acid concentration. The protonation of polyaniline is retarded in NMP compared to DMSO due to prevalent hydrogen bonding.