IT is not often that evidence in the form of steric hindrance is available for confirming the configurations of geometrical isomers among ethylenic compounds. The following observations are thus of interest. cis--Dibromocinnamic acid1 (m.p. 100°) esterifies to the extent of only 6 per cent in one hour, and 33 per cent in 16 hours, by the Fischer-Speier method, whereas the trans-acid1 (m.p. 136°) is esterified completely in one hour. This is in conformity with my earlier observation2 in regard to the different modes of addition of bromine to phenylpropiolic acid and its methyl ester.