Photoisomerisation of (2E)- and (2Z)-3-methyl-1-phenylbutane-1,2-dione 2-oxime (MPBDO) in several solvents was studied. With increasing dielectric constants of solvents, kinetic constants of forward reactions (E-form-->Z-form) did not change appreciably but those of reverse reactions (Z-form-->E-form) decreased. The positive correlation was found between equilibrium constants of photoisomerisation and dielectric constants of solvents.