Method for the purification of 3,9-bis{2-[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl} -2,4,8,10-tetraoxaspiro[5.5]undecane, to form crystals having a crystalline structure which, when subjected to Cu-K.sub..alpha. X-ray diffraction, show sharp X-ray diffraction peaks at angles of diffraction, to 2.theta.=2.8.degree., 2.theta.=8.7.degree. and 2.theta.=11.7.degree., by recrystallizing the ester at a temperature lower than 40.degree. C. using as a recrystallization solvent, A C.sub.5 -C.sub.10 alicyclic hydrocarbon, which may contain a limited amount of certain other solvents, or using a mixed solvent of a C.sub.5 -C.sub.10 aliphatic hydrocarbon and at least one other certain solvent in a certain weight ratio.