Gas-phase deposition of pure acetonitrile at around 100 K provides an X-ray amorphous solid the molecular constituents of which are identified by far IR – spectroscopy as antiparallel dimers. On increasing the temperature two irreversible phase transitions – from the amorphous to the high temperature modification and from the high temperature to the low temperature modification – were observed below 210 K by X-ray diffraction. At higher temperatures the well known equilibrium transition from the low temperature to the high temperature modification occurs. An attempt is made to discuss the mechanisms of the irreversible phase transitions in the light of the Classical Nucleation Theory and of Ostwald’s step rule.