The thermal decomposition of PH2BH2·NMe3 (1) in closed system was studied by static tensimetry with membrane null‐manometer. Compound 1 is unstable already at room temperature and partially decomposes with the release of gaseous trimethylamine and nonvolatile polymeric material. Storage of 1 at 311 K for 53 days results in evolution of small amounts of hydrogen and phosphine. At temperatures up to 329 K, PH3 evolution and BH3NMe3 adduct formation is observed after 3 days, and above 387 K, considerable amount of hydrogen gas is released. The data obtained indicate that the pyrolysis of 1 in the closed system may result in the enrichment of the condensed phase with boron due to phosphine loss.