The microstructure of thin poly-[2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) films was investigated by photoluminescence measurements and X-Ray diffraction analysis over thickness from several tens up to several hundreds of nanometer. Obtained results were correlated with exciton diffusion length obtained by the photovoltage method. All measurements revealed that there is a threshold thickness for the microstructural order as well as for exciton diffusion length which are primary physical properties for function of any device made up from such films. Development of more ordered phase can be observed above the critical thickness of about 150 nm. Below the threshold, the polymer chains are preferentially in conformations promoting localized intrachain interactions. Above the threshold, two main features can be observed: (i) the conjugation length of polymer chain segments reaches its maximum and (ii) polymer stacking in more ordered and larger crystalline domains influences strongly the material properties, namely enhances the exciton diffusion length. (C) 2014 Elsevier Ltd. All rights reserved.