Molecular beam infrared–radiofrequency double-resonance spectroscopy of [35Cl]chloromethane has been demonstrated, using laser Stark spectroscopy to tune rovibrational levels of this molecule into resonance with fixed-frequency CW CO2 laser radiation. Radiofrequency transitions in the 1–10 MHz region are observed with linewidths of ca. 20 kHz, close to the transit time limit of the apparatus. 18 ground state and 12 ν6= 1 vibrational state transitions have been assigned and measured. The upper-state transitions are fitted to the energy expressions in the total Stark–quadrupole Hamiltonian matrix, to obtain estimates of the ν6= 1 spin–rotation interaction constants for [35Cl]chloromethane. The results from this work have been combined with a previous study by Man and Butcher to produce values of CN=–6.1 (±4.8) kHz and Ck= 11.6 (±8.3) kHz. Also, estimates for the quadrupole centrifugal distortion coefficients have been obtained, including the previously undetermined Hougen distortion term, χD. The values obtained are χJ=–0.55 (±0.67) kHz, χK=–0.98 (±2.93) kHz and χD= 2.9 (±2.2) kHz.