PURPOSE: To isomerize cis-hexahydroterephthalic acid to the transisomer, economically, by heating an alkali metal or alkaline earth metal salt of cis- or cis/ trans-hexahydroterephtalic acid in solid phase in the presence of an alkali metal or alkaline earth metal hydroxide.

CONSTITUTION: cis- or cis/trans-hexahydroterephthalic acid (HTP) alkali metal salt or alkaline earth metal salt is heated in solid phase in the presence of preferably about 0.1 equivalent, based on said salt, of the alkali metal hydroxide or alkaline earth metal hydroxide to obtain trans-HTP. The alkali metal and alkaline earth metal of the above salt or hydroxide is e.g. Na, K, Ca, Ba, etc. The present process enables the isomerization of cis-HTP to trans-HTP in high yield by the heating at a low temperature for a short time compared with the conventional isomerization method, and facilitates the treatment of the isomerization reaction product.