of some aliphatic α-ketoacids takes place in water and some organic solvents by the catalysis of hydrogen ions. Trans isomer is more stable than cis isomer for lower keto acids and the reverse is the case for higher keto acids. The fraction of cis isomer in isomeric mixture at equilibrium in organic solvents such as ethyl acetate and ethyl ether, saturated with 6N hydrochloric acid, was larger than