Thick films (approximately 100 mu m) of NO2/N2O4 were prepared in a combined ultrahigh-vacuum ESR apparatus by adsorption of gaseous NO2 through a dosing system on a single-crystal surface (NiAl(110)). The adsorption temperature was in the range from 35 to 65 K. The line shapes of the ESR spectra are dependent on the adsorption conditions and show remarkable changes in heating processes. These observations can be explained in terms of geometrical rearrangements and molecular dynamics of the NO2 monomers. The line widths vary from 2 to 10 G depending on the concentration of ESR-active molecules through magnetic dipolar interactions. In all cases the observed line shape itself cannot be described by a simple distribution of randomly oriented molecules. The analysis of the spectra indicates a preferred molecular orientation with the NO2 C-2 axis perpendicular to the surface plane of the substrate. The spectra are fitted by computer simulation using a line width that depends on the z component of the nuclear spin as well as on the orientation of the NO2 molecules relative to the external magnetic field. The latter is thought to be needed according to effects uf molecular motion. The line shape of a sample annealed to 110 K is very well described by a statistical distribution of the molecules. The computer simulations an based here on a mixture of rigid particles and molecules undergoing rotational diffusion. The spectra recorded in the temperature range from 48 to 150 K indicate that the principal axis perpendicular to the molecular NO2 plane is the axis of rotation. Rotational constants are found to be in the range from 1.6 x 10(7) to 5.8 x 10(7) s(-1).