The present paper deals with important differences in the distribution of free volume in high and low free volume polymers as indicated by a joint investigation utilizing molecular modeling and positron annihilation lifetime studies. The main focus of this paper is on the molecular modeling approach. The polymers in question are the ultrahigh free volume polymer poly(1-(trimethylsilyl)-1-propyne) (PTMSP) and two polystyrene derivatives containing Si and F. Extended equilibration procedures were necessary to obtain reasonable packing models for the polymers. The transition state Gusev-Suter Monte Carlo method was utilized to prove a reasonable agreement between simulated and measured diffusivity and solubility values for the model structures. The free volume distribution was analyzed for the validated packing models and compared with respective positron annihilation data. In both cases, a bimodal distribution of free volume was observed for PTMSP while the polystyrene derivatives as other conventional glassy polymers showed a more or less monomodal behavior. Good qualitative agreement is demonstrated between size distributions of free volume elements in these polymer's obtained via molecular computer modeling and experiments using positron annihilation technique.