Abstract: AIMTo study the thermal stability, decomposition process and kinetics of such purine pharmaceuticals as aciclovir (Acv), penciclovir (Pcv), and their parent substance, guanine. METHODSUsing infrared technique, accelerating test method and thermogravimetry to investigate the thermal decomposition processes and using Coast-Redfern method, MKN method and Ozawa method to deal with the data to get kinetic functions. RESULTS The decomposition process and the formed products were derived, the kinetic model function was suggested by comparison of the kinetic parameters. CONCLUSIONPcv and Acv's degrading product for the first step is guanine. The sequences of their thermal stabilities is: Pcv>Acv. The two drugs' kinetic equation of thermal decomposition is expressed as: dα/dt=Ae^-Ea/RT2(1-α)^3/2.