We studied the reactions of 4-tert-butyl-4′-methoxydibenzoylmethane (avobenzone), the most common UV filter in formulations of sunscreens, under the conditions of disinfection. Three chlorinated compounds were identified as the main products. Their structures were confirmed by high-performance liquid chromatography-tandem mass spectrometry (HPLC–MS2) with accurate mass measurements and by gas chromatography–mass spectrometry (GC/MS). It was reliably proved, that the substitution involved the double bond of the enolic form of the diketone, rather than the activated aromatic ring, leading to the formation of 2-chloro-1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione and 2,2-dichloro-1-(4-tert-butylphenyl)-3-(4-methoxyphenyl)-1,3-propanedione as the main products. The third identified product was 2-chloro-1-(4-methoxyphenyl)ethanone.