Thin spin-cast films of poly [2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) were prepared from samples whose weight-average molecular weight (MW) was varied in the range of 10-1600 kg/mol. We have characterized the films by means of transmission and reflection ultraviolet-visible-nearinfrared (UV-vis-NIR) spectroscopy to derive the linear optical constants, and third-harmonic generation spectroscopy with variable laser wavelengths to get the modulus and phase angles of the complex third-order nonlinear optical susceptibility chi((3)). Increasing molecular weight yields films with significantly larger chi((3)) values, absorption coefficients, and refractive indices. The chi((3)) values of films from the largest and lowest MW differ by a factor of 4, which is caused by chain orientation effects, local field effects, and changes of the effective conjugation length.