The metastable hexagonal form of MoO3 has been successfully prepared starting from the ammonium molybdate, using a "Chimie Douce" reaction with NO2 gas. The lattice parameters of the as-prepared material were determined by XRD analysis to be : a = 10.633(2) Angstrom, and c = 3.719(1) Angstrom. The composition of this material obtained by physical and chemical analysis is MoO3, 0.09 H2O, showing that a small amount of water molecules remained while there were no more ammonium ions in this structure. The structural transformation from hexagonal to stable orthorhombic alpha-MoO3 phase was observed at 350 degrees C. It was also shown that NO2 reaction is a very useful method to remove topotactically ammonium ions from a parent structure at low temperature. This "Chimie Douce" process appears to be very promising for obtaining new metastable oxides with large tunnels or interlayer distances.