Studies were made of oral films formed in vivo, which had been allowed to form on fused silica and Ge-prisms during periods between 2 s and 2 h using a variety of physico-chemical methods. To produce surfaces of different qualities the silica and Ge-prisms had either been detergent-washed, glow discharge treated or covered with polydimethylsiloxane. The following simultaneous analytical techniques were performed on the adsorbed films: a. internal reflection infrared spectroscopy, b. ellipsometry, c. contact potential measurements, d. contact angle measurements, e. scanning electron microscopy and f. energy-dispersive x-ray analysis. The results of these studies show that the formation of oral films proceeds at high speed and is of a certain qualitative selectivity. The formed films were found to be stable over long periods of time, and only showed patches of adhering micro-organisms on some of the prisms which had been exposed in the oral cavity for 2 h.