Co/Mo multilayers with various low Co layer thickness and fixed Mo layer thickness were prepared by alternate evaporation. Transmission electron microscopy, X-ray diffraction and vibrating sample magnetometry were used to study the magnetic properties and microstructures of the films. The magnetic transition from ferromagnetic to superparamagnetic was observed at a critical thickness, accompanied with the structural evolution of the films from crystalline to amorphous. The contribution of the amorphous microstructure to the magnetic properties is discussed. (C) 2003 Elsevier B.V. All rights reserved.