In (CNMR)-C-13 spectra, four signals of two trans and two cis conformers in caprylolactarn were found in a mixed solvent of water and methanol. NMR line broadenings and coalescent phenomena in two conformers occurred at higher temperatures, giving high activation free energy (Delta G(double dagger)) for the isomerizations between two trans and cis conformers. Delta G(double dagger) are greatly influenced by the mixed ratio between water and methanol.