Mobility of water in polyheteroarylene (Matrimid®) was simulated at 300 K and different concentrations of water (0.5 wt.%-3 wt.%). Parameters of anomalous diffusion were calculated from molecular dynamics simulations on the base of mean square displacements of water molecules. It was found that mobility of water has nonmonotonic dependence on its concentration. Lower diffusion rates at concentrations below than 1 wt.% can be attributed to the sorption of water onto the polar groups of polymer (sorption sites). Decreasing of diffusion rate with increasing of water concentration is due to the formation of clusters of water molecules, which hampers the penetration of water between polymer chains.