The structure of the derivative formed during the reaction of 1,3-dimethyluric acid, the major metabolite of theophylline in humans, with iodopropane was elucidated. GC-MS analysis indicated the presence of a single dipropylated product even though three positional isomers are possible. The stability of the product was lower than anticipated since one of the alkyl groups added was readily removed by acid hydrolysis. Carbon-13 NMR proved to be the most useful technique for establishing the positions of alkylation. Assignment of carbon-13 resonances were consistent with placement of propyl groups onN-7 and on the oxygen atC-8. The structure of the product was assigned as 1,3-dimethyl-7-(1-propyl)-8-(1-propyloxy)xanthine. ThisO-alkylated structure is unusual since most heterocycles that can exist as lactim-lactam tautomers alkylate primarily on ring nitrogens.