A new borohydride, [CH3NH3](+)[BH4](-), has been synthesized through the metathesis of CH3NH3F and NaBH4 in methylamine. Room-temperature X-ray diffraction studies have shown that [CH3NH3](+)[BH4](-) adopts a tetragonal unit cell with considerable hydrogen mobility similar to that observed in NH3BH3. The kinetics and thermodynamics of hydrogen release have been investigated and were found to follow a similar pathway to that of [NH4](+)[BH4](-). Decomposition of [CH3NH3](+)[BH4](-) occurred slowly at room temperature and rapidly at ca. 40 degrees C to form [BH2(CH3NH2)(2)](+)[BH4](-), the methylated analogue of the diammoniate of diborane. The decomposition has been investigated by means of in situ X-ray diffraction and solid state B-11 NMR spectroscopy and occurred in the absence of any detectable intermediates to form crystalline [BH2(CH3NH2)(2)](+)[BH4](-)center dot[(CH3)(2)NH2](+)[BH4](-) and [BH2{(CH3)(2)NH}(2)](+)[BH4](-) have also been synthesized through analogous routes, indicating a more general applicability of the synthetic method.