Thermal degradation of quaternary hypoxanthinium salts was achieved by heating their solutions in dimethylformamide. 1,3-Dialkylhypoxanthinium bromides or iodides lose the 3-substituent as alkyl halide. The latter then attacks the imidazole ring at N-7 or N-9. Thermolysis of 1,3-dimethyl-6,8-dioxotetrahydropurinium iodide proceeds along either of two pathways: (i) loss of the 3-methyl group as methyl iodide, leading to 7,9-dihydro-1-methylpurine-6,8-(1H)-dione; or removal of hydrogen iodide to give the corresponding betaine, which is methylated at N-9 to give the 1,3,9-trimethyl-6,8-dioxotetrahydropurinium salt. The latter in turn decomposes to 7,9-dihydro-1,9-dimethylpurine-6,8(1H)-dione. Similarly 7,9-dimethylhypoxanthinium iodide (16) is degraded mainly, by loss of methyl iodide, to 9-methylhypoxanthine, accompanied by a small amount of the 7-methyl isomer. Compound (16) can also lose hydrogen iodide to give the corresponding betaine, which suffers methylation at N-1 to give the 1,7,9-trimethylhypoxanthinium salt. The latter again undergoes thermolysis to give a mixture of 1,7- and 1,9-dimethylhypoxanthine.