Abstract Rotational transitions of the vibrational ground state of cis-methyl nitrite have been assigned up to J = 20 over the frequency range from 12 to 40 GHz. Rotational constants, centrifugal distortion constants and quadrupole coupling constants have been fitted to the measured transition frequencies. In addition, rotational transitions in singly and doubly excited states of the NO and CH 3 torsions and in the singly excited state of the CON deformation mode have been assigned. Many of the absorption lines exhibited splittings due to the internal rotation of the methyl group. The internal rotation barrier has been determined to be V 3 = 734(2) cm −1 from the ground state splittings. Relative intensity measurements have yielded a transition frequency of 210(31) cm −1 for the No torsion. Furthermore, the energy difference between the more stable cis conformer and the trans conformer was found to be 314(22) cm −1 from the temperature dependence of the intensities of rotational transitions.