Amorphisation of Co-Mo magnetic thin films has been studied by using X-ray diffraction technique, and thermodynamic calculation based on Miedema's semi-empirical model has been used to clarify the nature of amorphisation in this binary alloy system. The transition composition from the hexagonal cubic packed structure to the amorphous (H/A) is about 19 at.%Mo when the Co-Mo thin films are deposited at room temperature (RT) under a working Ar pressure of 0.2 Pa and a deposition rate of about 0.7 angstrom/s, but increases up to about 23 at.%Mo when deposited at 300 degrees C. The calculated transition composition at RT based on the Miedema's model or the thermodynamically assessed parameters is 18.7 at.%Mo or 16.1 at.%Mo, respectively, well consistent with the experimental value of similar to 19.0 at.%Mo. Higher deposition rate and lower pressure shift the H/A transition composition to a higher Mo content. (C) 2009 Elsevier B.V. All rights reserved.