A new UV filter, the 1-(4-tert-butylphenyl)-2-decanyl-3-(4'-methoxyphenyl)-propane 1,3-dione called C10-DBM, was prepared by grafting a ten-carbon aliphatic chain to the alpha-carbonyl position of 4- tert -butyl-4'-methoxydibenzoylmethane (BM-DBM).(1) UVA absorption efficiency of a cosmetic preparation containing this new filter, called C10-DBM, was tested and compared to an identical preparation containing BM-DBM. The two preparations were irradiated under a 150-W xenon lamp or exposed to natural sunlight. The originality of this new filter resided in that its UVA absorbance appeared during the irradiation of the molecule. Moreover, although the molar absorption coefficient of C10-DBM in the UVA domain was lower than that of BM-DBM, its absorption showed much more photostable behavior under both methods of irradiation. After two hours of sunlight exposure, the preparation containing the BM-DBM lost 85% of its UVA absorbance, whereas the UVA absorbance of the preparation containing C10-DBM showed a decrease of 3% in comparison to the maximum absorbance obtained after 30 minutes of irradiation. Also, after two hours of exposure to natural sunlight, the UVA absorbance of the preparation containing C10-DBM remained above its initial value (before the irradiation began).