The phase behavior and self-association of omega-heptanolactam in supercritical chlorotrifluoromethane was studied from 322 to 404 K and up to 100 MPa. All measurements were performed using high-pressure near-infrared spectroscopy according to the analytical method. For the measurements, an optical high pressure cell fitted with two pairs of sapphire windows and fibre optics was used. The total substance concentrations were obtained from the first overtone region of the C-H stretching vibration (1600-1760 nm). The concentrations of the monomer species were determined from the first overtone of the N-H stretching fundamental (1450-1500 nm). No N-H absorption bands of dimerized species appear in the spectral range of the experiments. All concentrations were estimated from integrated absorbances using Lambert-Beer＇s law. The determination of the absorption coefficients corresponding to-infinitely dilute solutions turned out to be problematic. Various other methods of evaluation are presented and discussed. From the equilibrium constants determined, thermodynamic parameters of the association reaction are obtained.