The first use of a double‐pass geometry to enhance the conversion efficiency of a parametric nonlinear process in the tight‐focusing limit is reported. For third‐harmonic generation in liquid–CO‐O2mixtures using a CO2laser pump source, the observed enhancement is 2.5; elimination of reflection losses will result in an enhancement of 4. The double‐pass geometry has also been used to obtain accurate measurements of the wave‐vector mismatch &Dgr;kof the nonlinear medium.