New results of molecular modelling investigations on the transport of different small molecules in polyimides and polysiloxanes are discussed. The transition state Gusev-Suter Monte Carlo method reveals a reasonable coincidence between simulated and measured diffusivity and solubility values for the polyimides. A comparison between simulation data obtained for flexible chain polymers like poly(dimethyl siloxane) and polyimides shows that the lifetime of temporarily open channels is considerably longer for the stiff chain polymers than for the flexible chain polymers. This is probably a cause for the observed high rate of immediate back-jumps of permeating small molecules during diffusion processes in the polyimides. For amorphous poly(1-(trimethylsilyl)-1-propyne) (PTMSP), equilibrated packing models could be produced with a considerably higher density (1.22 gcm(-3)) than measured for respective polymer films. This indicates that the low density of the experimentally investigated PTMSP films (approximate to 0.7-0.8 gcm(-3)) is not the intrinsic ’equilibrium’ density but rather caused by the specific membrane preparation conditions. Low density PTMSP, unlike polyimides or polysiloxanes for example, seems to have a microporous morphology.