Experimental phase diagrams for the sealed systems low-density polyethylene (LDPE) - n-decane and low-density polyethylene - n-tridecane were plotted for the first time in a wide temperature concentration range using an original optical method and gravimetry. Liquid-liquid phase separation of LDPE and n-decane with the lower critical solution temperature of 280 degrees C was detected. Two branches of the solubility curve were constructed, which describe the temperature dependence of the component ratios in the semicrystalline LDPE with absorbed n-decane and in the liquid n-decane containing extracted polymer. In the domain between the solubility curve and polymer full amorphization curve the system LDPE - alkane can be identified with a single-phase microheterogeneous gel with crystallites as physical crosslinks. Below the solubility curve the system is two-phase but the true equilibrium between the phases cannot be established without destroying polymer crystallites. An empirical approach based on analyzing the parameters of the polymer full amorphization curve was applied to find that n-decane is a slightly better solvent for LDPE than n-tridecane. (C) 2018 Elsevier B.V. All rights reserved.