The absorption and emission spectral properties of thioflavin T (TFT ) in Nafion (Nf) and cellulose matrices have been studied. Formation of the emittive dimer is observed in both matrices. The monomer TFT emission is blueshifted in Nafion membrane (Nf), whereas it is redshifted in cellulose membrane when compared with the emission in aqueous solution. The dimer emission of TFT in the Na -Nf membrane undergoes off–on switching with acids and alkalis. The TFT molecule undergoes protonation in the H -Nf and the protonated dye is fluorescent. The dimer emission of TFT is not observed in the dry H -Nf membrane because of the protonation of the TFT molecule. The diffusion coefficient and the free energy of hydrophobic interaction for the TFT molecule in the Nf membrane are calculated. The TFT molecule experiences hydrophobic and electrostatic interactions in the Nf matrix, whereas it experiences a polar environment in the cellulose membrane. The 3D emission spectral studies support the formation of the emittive dimer in both Nf and cellulose matrices.