A p-dimethylaminobenzenesulfonate (MSE) polycrystalline powder is converted to the zwitterion p-trimethylammoniumbenzenesulfonate (ZWT) by chemical thermally induced reaction at T = 55,5°C. This reaction is associated to a first order ferroelastic phase transition in partially reacted MSE crystals. A13C-NMR study made during the in situ transformation MSE → ZWT shows that the reaction is of first order. A strong change appears in the low frequency Raman and Brillouin spectra of MSE monocrystals when passing the transition, but only a continuous evolution in the intensity of the high frequency modes is observed. Birefringence observations permit to specify the evolution of the optical axes position during the chemical reaction. The importance of stress induced by the reaction is discussed in relation to the phase transition.