The thermal isomerization reaction from methyl p-dimethylaminobenzenesulfonate (MSE) to p-trimethylammoniumbenzenesulfonate zwitterion (ZWT) in the crystalline state has been studied by optical crystallographic methods. Clear evidence of the lattice transformation occurring at similar to 60% conversion is provided by direct polarized light observations and by measurements of the birefringence Delta n(bc), and the optic axial angle 2V. Until the transformation the monoclinic MSE lattice remains optically homogeneous and highly crystalline. During the whole reaction course the orientation of the optical indicatrix is constant, within one degree, but the parameters of the indicatrix progressively and markedly change.-The direction of the highest refractive index corresponds, both for the starting MSE lattice and finally formed ZWT lattice, to the long molecular axis. Crossed axial plane evolution to a substantial similar to 48 degrees was found. A wider use of the birefringence methods in investigations of solid-state chemical reactions is suggested.