The tetrahedral splittings in the P and R branches of the ν3 band of natural methane have been examined with Doppler-limited resolution using a difference-frequency spectrometer. The spectra obtained by this difference-frequency mixer are compared to recent high-resolution grating spectrometer studies of 12CH 4 and enriched 13CH 4. The resolution, selectivity and precision are improved over the conventional methods. The mixing spectrometer utilizes tunable, narrow linewidth infrared radiation generated in the nonlinear optical crystal, LiNbO 3 as the beat frequency between a CW argon ion and a tunable dye laser. This spectrometer covers the 2.2 to 4.2 μm infrared spectrum with an instrumental resolution of 5 × 10 -4 cm -1 and continuous scans up to ∼1 cm -1 and with ir power ∼1 μW.