Langmuir—Blodgett (L/B) films of tritiated fatty acid were prepared to measure the electron antineutrino mass. The measurement requires that a source should be highly radioactive, homogeneous, very thin, uniform, and nonvolatile in vacuo. With regard to the volatility, the L/B film of fatty acid is, in general, not very stable in vacuo, and desorption of the source molecules results in the main background rates of the measurement and reduces the sensitivity of the mass determination. In the present work, we examined the stability of the L/B film in vacuo, and tried to improve the stability of the film by (i) perfect formation and deposition of metal soap molecules that are more resistant against evaporation than the original fatty acid molecules, and (ii) cooling of the 3H source with liquid nitrogen in order to reduce the evaporation. Finally, a uniform and highly radioactive 3H source, which is nonvolatile even at a pressure of 10 −4 Pa, was obtained using the Langmuir—Blodgett film of cadmium [ 3H]icosanoate (sp act 200,000 Ci/mole).