A detailed study of the structure and the dynamics of the polystyrene-(PS-) ethylbenzene (EB) polymer-penetrant system is presented. The work combines dual scale (atomistic/coarse grained) simulations and experiments. United atom atomistic NPT molecular dynamics simulations as well as coarse-grained (CG) molecular dynamics simulations have been executed and the ability of the CG simulations to predict the dynamic properties of the polymer/penetrant system is examined. The results are directly compared to pulse-field gradient nuclear magnetic resonance measurements. The coarse-grained simulations, which are much faster than atomistic ones, are capable of describing the diffusion of EB molecules. This opens up the way to study polymer/penetrant systems difficult to reach by experiment but of technological importance.