Abstract.  The formation and generation of shocks is a topic of wide interest in many fields of physics, but the role of the kinetics effects and the properties of the particle distribution function across the shock front have not been explored in detail. Using particle-in-cell simulations to study electron-positron collisionless shocks we explore the features of the particle distribution in the upstream, downstream, and shock transition region, and the deviations to a Maxwellian distribution. The theoretical model developed to account for these effects shows that a strong tail can change the shock properties (shock velocity, jump conditions) significantly. However, in the standard case, the decrease of the upstream bulk speed has a bigger impact. These effects are illustrated with particle-in-cell simulations. The relevance of these results for astrophysical shocks is also discussed.