RESUMEN
Photochemical (cis-trans) isomerization of levorin A2 under the effect of light at wave lengths of 313 and 365 nm (excitation of the polyenic system and p-aminobenzoylene respectively) was studied. In case of the polyenic system excitation the reaction was reversible and the process quantum yield was extremely low while the use of light at the wave length of 313 nm as photochemical activator resulted in increasing the quantum yield by two orders. In this case triplet-triplet transfer of energy took place. A calculation procedure is recommended for quantitative determination of the quantum yield of levorin isomerization including direct and reverse reactions. The procedure may be used in study of other objects with similar mechanisms of phototransformation.