We have studied thin films of the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), prepared from polymer samples whose weight-average molecular weight (M-w) was varied in the broad range of 10-1600 kg/mol. Anisotropic refractive index measurements by means of waveguide prism coupling and reflectometry as well as polarized infrared spectroscopy were used to analyze the polymer chain orientation in the films. We found that the film morphology depends significantly on the molecular weight, especially in the range M-w < 400 kg/mol. Thin films of high molecular weight MEH-PPV have most polymer chain segments oriented parallel to the film plane-in contrast to low molecular weight samples which have nearly random orientation of the chain segments. Appropriate choice of molecular weight enables fine-tuning of the refractive index of slab waveguides and reduction of their mode propagation losses to less than 1 dB/cm.