Polarization modulation infrared reflection absorption spectroscopy was applied to investigate two-dimensional nanoclusters of CF3(CF2)(3)(CH2)(22)COOH (C27F9) on the aqueous Cd2+ subphase in a Langmuir trough at 288 K. The -(CH2)(22)- long axis was always inclined on the subphase and had triclinic parallel packing with compression from 0.60 to 0.30 nm(2). In contrast, behenic acid, without the CF3(CF2)(3)- tail, formed orthorhombic micrometer domains on the Cd2+ subphase with its hydrocarbon chain vertically aligned. It is concluded that the steric hindrance imposed by the bulky CF3(CF2)(3)- tail is responsible for the specific orientation and packing of the -(CH2)(22)-part as well as the formation of the nanometer-size clusters.