Plasma-enhanced chemical deposition processes have been studied by comparing the performance of flexible diffusion barrier layers on plastic films produced in the same reactor. Under similar experimental conditions, a higher deposition rate is achieved by microwave discharges than by bipolar, pulsed d.c. magnetron sputtering processes. However, with both discharge modes, dense hydrocarbon coatings were produced, exhibiting a barrier improvement factor up to 120 and a flexibility ranging from 1.1%-8.8% before formation of microcracks started to dominate permeation characteristics. The density of the coatings is 1.0-1.6 g cm(-3) and their hydrogen content varies from 23%-33%.