Thermostability and structure transformations upon heating of composite systems consisting of a microporous polyethylene film and a conducting polyaniline (PANI) layer have been investigated by thermogravimetry (TG), differential thermal analysis (DTA), differential scanning calorimetry (DSC), and wide-angle X-ray scattering (WAXS). Microporous polyethylene films were prepared by melt extrusion with subsequent annealing, uniaxial extension, and thermofixation. Polyaniline layers were formed by polymerizing aniline from solution in situ or by deposition of colloidal polyaniline dispersion on the polyethylene film surface. Composite systems have been found to demonstrate a considerably lower shrinkage upon heating than microporous polyethylene substrates. It has been discovered from DTA, DSC, and WAXS data that oriented phases in the polyethylene melt are formed in composite systems at melting temperature of polyethylene support. Oriented state in polyethylene melt is maintained at heating of composite samples up to the temperatures exceeding polyethylene melting point at several tens degrees.