The transformation of MoO3 into gamma-Mo2N during its nitridation by temperature-programmed reaction with NH3/has been investigated. The X-ray diffraction (XRD) results indicate that the conversion of the MoO3 bulk structure was significant at 673 K, although a notable increase in the surface area was observed at higher temperatures such as 773 K. For the sample synthesized at 973 K for 2 h, the achieved surface area was 140 m(2).g(-1) with approximately 60% pore volume within the diameter range of 20-30 Angstrom. The XRD and differential thermal analysis (DTA) results indicate that some intermediates, i.e., H0.3MoO3, MoO2, and gamma-Mo2OxN1-x, were formed during the pseudomorphic transformation of MoO3 to gamma-Mo2N in the temperature range of 673-973 K. Some new information about the transformation reaction was provided by X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared (FT-IR) spectroscopy. XPS results revealed that Mo2+ species was the main species exposed at the surface of passivated samples.