The phase behavior of binary mixtures of non-crystallizable racemic poly (D, L-lactic acid) (PDLLA) and the mosquito-repellent/drug N,N-diethyl-3-methylbenzamide (DEET) was analyzed with respect to the effect of the polymer molar mass on the liquid-liquid (L-L) phase separation characteristics, by cloud-point measurements and differential scanning calorimetry. The PDLLA/DEET system shows a subambient upper critical solution temperature (UCST), with the critical temperature decreasing and critical polymer concentration increasing with decreasing molar mass of PDLLA. The obtained L-L phase separation curves were used to estimate the temperature-dependence of the interaction parameter, confirming that the enhanced miscibility of the system components in case of low molar mass PDLLA is due to increased entropy of mixing.