The mechanism of trimethylamine borane (TMAB) hydrolysis in the presence of sulfonate cation exchanger in H+-form, H2SO4 and trimethylamine deuterosulfate has been established. The hydrolytic reaction is first order in TMAB when carried out in the presence of polymeric acid. The rate of TMAB hydrolysis is directly proportional to the cation exchange capacity of the resin. It was also established that the resin phase acts as a suppressor of TMAB hydrolysis. For the first time, it was noted that TMAB hydrolysis may be carried out when trimethylamine (TMA) is applied as a hydrolyzing agent. The initial effective rate of hydrolytic reaction in this case is much higher than that observed in sulfuric acid solution. Most importantly, the complete absence of isotope exchange between nitrogen-bound deuterium ions of TMA and hydride hydrogens of TMAB in alkaline and neutral media has been observed.