Abstract: The present paper reports a thermodynamic study for the substitution of tritium for hydrogen from vinylbenzene (VB)—one of the monomers of polystyrenedivinylbenzene (SDB) by means of the density function theory (DFT) B3P86 method at 6-311G base level. The thermodynamic functions are calculated at different temperature. The standard formation functions, the equilibrium constants and the equilibrium pressure ratios of tritium and hydrogen gas for monomer VB molecules at different temperature have been calculated. The calculation results show that the elevation of temperature does not favor the substitution reaction of T₂(g)+SDB_(H2)(s)→H₂(g) +SDB_(T2)(s). This conclusion is in good agreement with some experimental investigations.