Considerable progress in the development of new materials is achieved due to the modification of polymer materials by carbon nanoparticles of various kinds. In this work new composites of nanodiamonds (ND) and poly (phenylene oxide) (PPO) were prepared by solid phase dispersing. Their solutions and dense nonporous films were studied by different techniques. The structure of the PPO/ND composites was examined by atomic force microscopy, dynamic light scattering, and birefringence. The increase of ND content in the film leads to a more complex structure because of accumulation of aggregates containing ND particles and PPO molecules. The physical and mechanical properties were exhaustively evaluated by measuring intrinsic viscosity, density, surface tension, the Young's modulus, yield stress, tensile strength, and ultimate strain. Gas transport properties were determined by measuring H ₂, CO ₂, N ₂, CH ₄, and O ₂ permeation. The gas permeability coefficients decrease slightly with an increase of the ND content in composites, but the ideal selectivity of gas separation increases at the same time. POLYM. COMPOS., 39:3952–3961, 2018.