By means of fluorescence single molecule spectroscopy, it is shown that the conformation of high molecular weight conjugated polymers in thin films depends on the nature (chemical composition and molecular weight) of the host matrix. When a high molecular weight poly(p-phenylenevinylene) derivative (OC1C10-PPV) is dispersed in a thin polyvinylbutyral-co-vinyl alcohol-co-vinyl acetate (PVB) film, the spectroscopic properties suggest that OC1C10-PPV is in a highly coiled conformation. On the other hand, in a thin polystyrene film (PS) of low molecular weight, the OC1C10-PPV chains adopt a more extended conformation. Increasing the molecular weight of polystyrene leads to increased coiling of the OC1C10-PPV chains, related to poor solvation. The film preparation conditions (spin-coating or drop-casting) have no effect.