The non-linear optical properties of poly[1,4-phenylene-1-phenyl vinylene], poly[1,4-phenylene-1,2-di(4′-phenoxyphenyl) vinylene], poly[2,5-dioctyloxy-1,4-phenylene vinylene] and poly[2- methoxy -5- (2'-ethylhexyloxy)-1,4-phenylene vinylene] MEH-PPV with different molecular weights were studied by the Third Harmonic Generation (THG) technique. The dispersion of the χ3 coefficient was measured in the wide infra red range from 1.2 up to 2.1 μm. It was shown that the highest χ3 value, observed for the high molecular weight MEH-PPV (Mw 230.000), reaches the maximum of 8x10-11 esu at 1.77 μm. At the same time, the medium molecular weight MEH-PPV (Mw 95.000) demonstrated not significantly different χ3 value of 6x10-11 esu. The dispersion of the χ3 coefficient was compared to the absorption of the third harmonic beam and strong χ3 enhancement relative to the excitonic absorption was observed. However, a surprising shift of about 70 nm towards the longer wavelengths between the two peaks was always observed for all studied PPV derivatives. The results of THG measurements were compared to those obtained by conventional Zscan technique in the same range. It was shown that the THG χ3 values in infrared range were up to one order of magnitude lower than those of direct Z-scan measurements.