At room temperature, monolayer films of poly(dimethylsiloxane) spread on the air-water interface display nearly constant surface pressures in the region between surface concentrations of 0.75 and 1.6 mg/m(2). Studies of these films in this region by epifluorescence microscopy and external reflectance infrared spectroscopy are reported. Two surface phases were detected by the epifluorescence micrographs. The infrared reflectance spectra in the 1000-1100 cm(-1) region and at concentrations between 0.75 and 1.6 mg/m(2) can be fit as a mixture of infrared features observed at these surface concentration limits. At 0.75 mg/m(2), the infrared reflectance data are consistent with a random chain in which the oxygen atoms are largely exposed to the water surface, described by earlier workers as a twisted caterpillar, or a two-dimensional random coil with as many oxygen atoms as possible exposed to water. At 1.6 mg/m(2), the infrared data indicate that the oxygen atoms are largely excluded from contact with water and suggest the presence of a more ordered structure consistent with helical segments which have the helix axis parallel to the water surface.