High-resolution Fourier-transform spectra of isotopically natural pure chlorine nitrate, ClONO2, were recorded with a resolution of 0.00094 cm-1 at temperatures of 190 and 297 K. An extensive vib-rotational analysis of the ν4 and ν3 fundamental bands as well as the hot band, ν4 + ν9 - ν9, of the most abundant 35Cl isotopomer was performed for the 190 K spectrum. In addition, the cold bands of the lesser abundant 37Cl isotopomer were partially assigned. Accurate upper state rotational Hamiltonian constants were derived. In particular, the following band centers were determined: ν0(ν4) = 780.2161(8) cm-1, ν0(ν3) = 809.7513(8) cm-1, ν0(ν4 + ν9 - ν9) = 778.7235(8) cm-1 for 35ClONO2 and ν0(ν4) = 778.8695(8) cm-1 for 37ClONO2. These constants were used to generate spectral parameters that permit a precise modeling of experimental cross sections of the ν4 fundamental Q-branch of 35ClONO2 over the temperature range of 190--297 K and atmospheric pressures up to 150 mbar. This Q-branch is used for atmospheric retrievals.