Liquid methylhydrazine monohydrate (CH3NHNH2 . H2O; molar mass = 0.06409 kg mol(-1) ) can be easily quenched into a glassy state. On reheating, the glass transition occurs near 160 K (T-g) with an important heat capacity variation (Delta C-p=96.8 J mol(-1)K(-1)). This glass is extremely "fragile" according to Angell’s classification. Depending on the thermal treatment, this glass transition is followed by successive phase transformations with slow and complex kinetics which are described here in detail. Thermodynamic properties were investigated by means of adiabatic calorimetry. Enthalpies and heat capacities were determined or extrapolated from 100 K to 265 K for the glassy state and all condensed phases : the liquid, the undercooled liquid, the metastable and the stable solid phases. The melting temperature T-fus,T-s and the melting enthalpy Delta H-fus,H-s for the stable solid phase were estimated to be 234 K and 15.4 k J mol(-1), respectively. The temperature evolution of the enthalpy for the metastable solid phase was investigated using previous measurements for the metastable solid phase : T-fus,T-m 219 K and Delta H-fus,H-m = 9.8 k J mol(-1). The spontaneous evolution of the temperature of the undercooled liquid was followed from 188 K to 222.4 K under adiabatic conditions. This is interpreted in terms of phase transformations and kinetics. Three different processes can be considered. The first process leads to the quasi-complete transformation into the metastable solid phase from the undercooled liquid. Then the crystallization of the stable solid phase starts from the remaining undercooled liquid and further from the metastable solid phase. The third process corresponds to the melting of the metastable solid phase at 219 K and the rapid and simultaneous crystallization into the stable solid phase. At the end of the process, the complete transformation of the stable solid phase is observed. : the liquid, the undercooled liquid, the metastable and the stable solid phases. The melting temperature T-fus,T-s and the melting enthalpy Delta H-fus,H-s for the stable solid phase were estimated to be 234 K and 15.4 k J mol(-1), respectively. The temperature evolution of the enthalpy for the metastable solid phase was investigated using previous measurements for the metastable solid phase : T-fus,T-m 219 K and Delta H-fus,H-m = 9.8 k J mol(-1). The spontaneous evolution of the temperature of the undercooled liquid was followed from 188 K to 222.4 K under adiabatic conditions. This is interpreted in terms of phase transformations and kinetics. Three different processes can be considered. The first process leads to the quasi-complete transformation into the metastable solid phase from the undercooled liquid. Then the crystallization of the stable solid phase starts from the remaining undercooled liquid and further from the metastable solid phase. The third process corresponds to the melting of the metastable solid phase at 219 K and the rapid and simultaneous crystallization into the stable solid phase. At the end of the process, the complete transformation of the stable solid phase is observed.