Abstract
(1) No isomerization of isostilbene to stilbene occurs with hydrogen chloride in the presence of either reduced nickel or oxygen. (2) Hydrogen bromide and hydrogen chloride cause the isomerization of dimethyl maleate to the fumarate either in absence of solvents or in carbon tetrachloride solution. (3) Oxygen has no effect on the isomerization of dimethyl maleate by hydrogen halides. (4) The isomerization of dimethyl maleate by hydrogen halides belongs to another type of catalyzed cis-trans isomerization apparently distinct from that of isostilbene by hydrogen bromide and oxygen. (5) A hypothesis has been proposed concerning the mechanism of the isomerization of dimethyl maleate caused by hydrogen halides. In conclusion, the author wishes to express his sincere thanks to Prof. Y. Urushibara for his kind guidance and encouragements throughout the work and to Prof. S. Mizushima for valuable advices. Thanks are also due to the Japan Society for the Promotion of Scientific Research (Nippon Gakujutsu Shinko-Kwai) and to Ozi Seisi Company for grants.

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