The surface modification of polytetrafluoroethylene (PTFE) by microwave plasma treatment was investigated by means of contact angle measurement and e.s.c.a. studies. Various gases (e.g. O2, O2/N2, NH3) were used. The influence of the various plasma parameters, such as power, gas flow, distance between the sample and the centre of the discharge, treatment time, etc., has been evaluated. No modification was induced by O2 and O2/N2 treatment, whatever the treatment conditions. NH3 plasma irradiation, however, rendered the PTFE surfaces more hydrophilic, leading to an increase of the polar component of the surface energy from 4.5 to approximately 57 mL m-2 under optimized treatment conditions. NH3 treatment led to defluorination, crosslinking, hydrocarbon (CC,CH) bond formation, and incorporation of nitrogen-containing groups, as confirmed by e.s.c.a. Oxygen was also detected at the surface of treated PTFE. Correlations between the contact angle, defluorination rate and surface nitrogen and oxygen contents, have been established. Optimization of operational NH3 plasma parameters, leading to the best wettability of the treated samples, is also reported,