The thermal isomerization reaction from methyl p-dimethylaminobenzenesulfon
ate (MSE) to p-trimethylammoniumbenzenesulfonate zwitterion (ZWT) in the cr
ystalline state has been studied by optical crystallographic methods. Clear
evidence of the lattice transformation occurring at similar to 60% convers
ion is provided by direct polarized light observations and by measurements
of the birefringence Delta n(bc), and the optic axial angle 2V. Until the t
ransformation 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 t
he indicatrix progressively and markedly change.-The direction of the highe
st refractive index corresponds, both for the starting MSE lattice and fina
lly formed ZWT lattice, to the long molecular axis. Crossed axial plane evo
lution 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.