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An increasing number of early-type (O and Wolf-Rayet) colliding wind binaries (CWBs) is known to accelerate particles up to relativistic energies. In this context, non-thermal emission processes such as inverse Compton (IC) scattering are expected to produce a high energy spectrum, in addition to the strong thermal emission from the shock-heated plasma. SIMBOL-X will be the ideal observatory to investigate the hard X-ray spectrum (above 10 keV) of these systems, i.e. where it is no longer dominated by the thermal emission. Such observations are strongly needed to constrain the models aimed at understanding the physics of particle acceleration in CWB. Such systems are important laboratories for investigating the underlying physics of particle acceleration at high Mach number shocks, and probe a different region of parameter space than studies of supernova remnants. 1. Colliding-wind binaries as particle accelerators More than 30 early-type stars (most of them confirmed or suspected binaries) exhibit synchrotron radiation in the radio domain (see De Becker 2007). This indicates that relativis-tic electrons are present, and therefore that a particle acceleration process is at work. The acceleration mechanism is most proba-Send offprint requests to: M. De Becker bly Diffusive Shock Acceleration (DSA) in the presence of strong hydrodynamic shocks (e.g. Pittard & Dougherty 2006). In massive CWBs, particle acceleration is expected to occur at the global shocks bounding the wind-wind collision region, and perhaps also within this volume (see Fig. 1). Such systems are further considered to be candidates for the production of cosmic-rays. In the context of CWBs, non-thermal X-rays are also expected to be produced. Previous observations with XMM sug