An experimental full phase diagram for the binary system low-density polyethylene thymol was for the first time constructed in a broad temperature-concentration range using an original optical method, turbidimetry, and differential scanning calorimetry. Limitations of the two latter techniques were critically analyzed. The diagram belongs to the type semicrystalline polymer thermodynamically poor solvent so that in the whole concentration range polyethylene and thymol play the role of a solvent and solute, respectively. Thermally induced phase separation leads to the formation of a capillary-porous body. Thymol crystals formed in its pores melt at lower temperatures than crystals from pure thymol solution. The data obtained by different methods can be used for developing the technology of fabricating films with antimicrobial properties.