Mo/Si multilayer systems with boron carbide (B4C) diffusion barrier layers were deposited on sapphire and silicon substrates by DC magnetron sputtering. Samples were subsequently annealed in vacuum at temperatures between 100 and 800 degreesC for duration of between 20 min and 30 h. Thermally stimulated solid state reactions have been characterized by X-ray analysis methods. Mo/Si multilayers without barrier layers are stable up to 100 degreesC. Interdiffusion was observed to start by 150 degreesC. It was found that B,C diffusion barrier layers with thicknesses between 0.3 and 1.0 nm, depending on the stack sequence, give rise to an increase of the thermal stability up to 400 degreesC. The impact of thermal treatments, at various temperatures and annealing times, on thickness and composition of the interdiffusion layers was investigated by X-ray reflectometry, wide angle X-ray scattering, cross-sectional high resolution transmission electron microscopy and fluorescence extended X-ray absorption fine structure measurements in combination with excitation of X-ray standing waves. The last method was used to investigate the short-range order of Mo/Si multilayers depth-resolved. (C) 2003 Elsevier B.V. All rights reserved.