Doppler-free two-photon rotational transitions J = 13 <--<-- 11 and J = 12
<--<-- 10 of OCS and J = 8 <--<-- 6 and J = 7 <--<-- 5 of CHF3 were detecte
d in the frequency range 134-156 GHz, using a novel, highly sensitive intra
cavity-jet technique. The sub-Doppler narrowing of the observed peaks (down
to 40 kHz full width at half maximum as compared to 300 kHz of the Doppler
width) demonstrates the potential of this new technique for high precision
millimeter wave spectroscopy. The possibilities of the further reduction o
f the two-photon absorption line widths are considered.