Growth conditions for the formation of thin films (100–300 Å) of potassium-doped superfullerides (KxC60, x≫6) are examined. Thin films of these compounds are formed by depositing C60 onto a potassium precovered single crystal quartz substrate maintained at 200 K or lower, in a proportion of K:C60≫12:1, followed by annealing the surface to the K-sublimation temperature (300 K). In situ measurements of electrical and optical properties are used to identify the compounds. The formation of superfullerides is confirmed by C60 doping of these phases to check for the formation of insulating K6C60 with a characteristic absorption spectrum. The absorption spectrum of the superfullerides shows distinct features corresponding to the filling of the t1g band. The presence of two superfulleride phases is suggested, a near-metallic superfulleride KxC60 (x≈11.2) and a more insulating KxC60 (x≈8–9). © 1998 American Vacuum Society.