A mixed oxide catalyst for the selective oxidation of ethane to ethene and acetic acid, and having the composition Mo(73)V(18) Nb(B)O(x) was prepared and characterised. The crystalline phases found were Mo6V9O40, Mo3Nb2O11, and MoO3; the amorphous part was shown to mainly have the composition Mo(84 +/- 3)V(13 +/- 3) Nb(2 +/- 0.5)O(x). The crystallinity was found to be very sensitive to the calcination temperature in the range 400 to 500 degrees C, where a large part of the amorphous phase is transformed into the crystalline phases given above. The crystalline phases found in the catalyst were prepared separately in pure form, as were additional selected phases from the oxidic system Mo-V-Nb. Analytical transmission electron microscopy was used to determine the contribution of the different phases in the catalyst to the total surface area. It appears that the amorphous phase, shown to consist of small grains (diameter approximate to 10 nm) and, thus, to have a high surface area, may dominate the properties of the catalyst.