Abstract The monoisotopic methyl chloride species 13 CH 3 35 Cl and 13 CH 3 37 Cl were synthesized and investigated by high-resolution (ca. 2.5×10 −3 cm −1 ) infrared ( J max =63) and millimeter-wave spectroscopy ( J max =50). Ground state parameters up to sextic centrifugal distortion constants have been determined by merging ground state combination differences with pure rotational data. The axial rotational constant A 0 was determined from allowed and perturbation-allowed infrared transitions observed in the ν 2 and ν 5 bands around a local crossing. Parameters of the excited states 3 1 and 6 1 were determined by jointly fitting infrared ( σ Fit ca. 5×10 −5 cm −1 ) and rotational data ( σ Fit ca. 55 kHz) using both an isolated band model and a model taking into account x , y -Coriolis resonance, with an interaction constant adopted from the harmonic force field. The 6 1 level is affected by (2, 2) and (2,−4) l -type resonance responsible for A 1 A 2 splittings of the kl =+1 and kl =−2 sublevels, respectively. The splitting of the P (PQR) 3 lines was resolved for J ⩾45.