Four‐wave mixing of the type &ohgr;4 = 2&ohgr;1 − &ohgr;2has been observed in pure liquid CO at 77 °K. The CO2pump beams were derived from separate resonators to yield a mixing signal at 8.64 &mgr;m. The power dependence of the mixing signal has been studied over four orders of magnitude and the nonlinear susceptibility has been measured. Scaling calculations show that efficiencies greater than 10% are expected for pump powers near the breakdown threshold and with confinement of the pump beams in a hollow dielectric waveguide.