The effect of orientation and crystallinity of certain polymers, polyethylene, polypropylene, polyethylene terephthalate (PET) and poly (ether ether ketone) (PEEK) , upon the extent and nature of plasma oxidation was studied. It was found that increasing the extent of surface ordering lessened susceptibility to plasma oxidation and reduced the subsequent decay of surface treatment. The surface ageing of plasma oxidised PEEK was extensively studied with regards to the transient increase in hydrophilicity that has been observed after plasma modification. The decay and transient increase in hydrophilicity were found to be dependent upon crystallinity and storage temperature. An estimate of the activation energies for processes leading to the increase in contact angle after plasma modification were calculated and found to suggest that these processes were rotational reorganisations at the surface as opposed to migrational reorganisations. The decay of other plasma modified surfaces revealed that plasma oxidised PET and plasma fluorinated PEEK both underwent transitional reorganisations at the surface, however no such change was observed for ammonia plasma treated PEEK. Plasma modification of carbon fibres was investigated with regards to improving composite performance. Microwave plasma treatments were found to be as good as standard commercial treatments. Graphitic carbon was investigated as a model for carbon fibre surfaces, however, the plasma modified surface was found to age more readily and to be too labile for useful comparison.