In this work we report on the infrared properties of the thermoelectric (1-x)PbTe-xPbSnS 2system with x=0.03, 0.06, 0.11 and 0.33. The results obtained by the analysis of the reflectivity spectra are discussed together with the structural and morphological characteristics obtained by XRD and SEM-EDS measurements. The system was found macroscopically homogeneous for x=0.03 and x=0.06 and phase separated for x=0.11 and x=0.33. The analyzed ∼150cm -1 PbS impurity mode demonstrated a composition close to the PbTe 0.98S 0.02 for the major phase. The incorporation of PbSnS 2 causes a reduction in plasma frequency (decrease in carrier frequency concentration) and an increase in carrier mobility.