[en] Detailed measurements have been made of degenerate four-wave mixing (DFWM) in a resonant homogeneously broadened gas. The measurements were performed in SF₆ using a CO₂ laser operated on the 10.4-μm branch. The experimental results were compared to a two-level theory for a resonant saturable absorber developed by Abrams and Lind. The measured value of 7% reflectivity on the P(20) line was in excellent agreement with Abrams and Lind when corrected for thermal motion. A peak reflectivity of 38% was observed for off-resonant operation on the P(8) line. In addition to the usual two-level nonlinear response, discussion and measurement of the coherent three-level nonlinearity is also presented. A two-photon contribution in SF₆ using the P(16) line of CO₂ equals the one-photon response. Initial observations of coherent propagation effects are also presented