Abstract: The distribution of tritium in 316L and 22 13 5 types of stainless steel cylinders after being exposed to tritium gases at the temperature of 673K and 1 013K and aged in air at room temperature for one to three years is determined with the liquid scintillation counters through etching the samples. The bulk distribution of tritium and helium 3 concentration in these samples is calculated, based on the diffusion equation and finite difference method. The calculation is in good agreement with the determination. According to the results of calculation and experimental determination, it can be concluded that the decaying of tritium to helium 3 is the main cause that has changed tritium concentration during the aging procedure due to the very low diffusion rate at room temperature. And because of the oxide layer on the surface, which can slow the diffusion of tritium, tritium near the surface only partly escapes from the samples. There are two forms of tritium——the “dissolved”and the “non dissolved”. The former is usually less than the latter, which enters the solution while the samples are being etched. The latter is captured by the crystal defects as molecules or clusters of many tritium atoms, and released as gases while being etched.