In the Belousov Zhabotinsky reaction, malonyl radicals formed during the oxidation of malonic acid by Ce4+ play an important role in the mechanism of the negative feedback loop. In the past, we have analyzed the end products in the Ce4+-malonic acid reaction applying HPLC technique. For comparison, we generated malonyl radicals by UV irradiation of solutions of malonic acid and we identified the reaction products. Two of these are the same as in the Ce4+-malonic acid reaction, but some additional products are also formed. To explain our experimental results, a new reaction path is proposed where malonyl radicals and hydrogen atoms are the first intermediates in the photochemical decomposition of the malonic acid. Mechanistic differences between this photochemical decomposition and the Ce4+-malonic acid reaction are also discussed.