Abstract Evidence is given for hindrance to thermal stereomutation of cis- and trans-perchlorostilbenes. Such hindrance is accounted for in terms of steric inhibition of resonance in the transition state of the isomerization due to the ortho chlorine atoms. Also, it is shown that under conditions leading to isomerization in closely related cases, i.e. photochlorination and nuclear perchlorination, the perchlorostilbenes are quite inert. These results are interpreted in terms of steric strain of the reaction intermediates and transition states.