An inverse Lamb dip has been observed for infrared‐microwave two‐photon transitions in14NH3and15NH3. The signal was sufficiently strong to be observed on an oscilloscope with a detector time constant of 3 msec. The observations establish a method of measuring differences between laser frequencies and molecular absorption frequencies with high accuracy even if this absorption is outside of the tuning range of the laser lines. A double resonance experiment involving the two‐photon inverse Lamb dip has also been conducted.