A capacitor the dielectric of which is a plastic film. The electrical properties depend on the molecular structure. Material made from polar (asymmetrical) molecules will have an increased dielectric constant that is frequency dependent; nonpolar (symmetrical) molecules give a material the properties of which are independent of frequency.

Two main types of plastic-film capacitor exist: polystyrene film and polyester film capacitors. The former contains a nonpolar plastic with excellent properties that is used with metal-foil electrodes to produce a low-loss capacitor with good capacitance stability. Polyester-film capacitors are slightly polar plastic-film capacitors with either foil or metallized electrodes and are useful for d.c. applications and for operation up to 125 °C.