Phase transitions in polymeric systems, e.g. melting, crystallization and liquid-liquid demixing, have been studied by means of various techniques: calorimetry, NMR, DMTA and optical methods (light scattering, turbidity). To study the optical properties of polymeric samples, an experimental apparatus was designed on purpose and set up. The sample is a thin film (10x20x0,1 mm), enclosed between two glass slides and a PTFE frame, in order to prevent polymer leakage. A very thin thermocouple was placed on sample surface for direct temperature measurement. This sample holder was inserted in between two aluminum slabs, equipped with a narrow slit for optical measurements. Sample temperature was controlled by means of electrical heaters embedded into the aluminum slabs. Sample was enlightened by a laser diode, whereas transmitted light was detected with a photodiode. All equipment was enclosed in a sealed chamber with a constant nitrogen flow to prevent sample degradation by oxidation. Measurements were carried out on PET and two different PA’s, tested as pure polymers and blends. The thermal history imposed to the sample consisted in a rapid heating from ambient temperature to a temperature below the melting point (100-150°C), a stabilization (10-20 min), and then a heating at constant rate (1°C/min) until around 300°C. After a second stabilization period (10-20 min), the sample was cooled at 1°C/min to 100°C. The data obtained were compared with DSC performed with the same thermal history. In correspondence with transitions detected via DSC, the optical signal showed a steep variation: crystallization resulted in a rapid decrease of transmitted light, whereas melting gave up an increase of transmitted light. Further variations were recorded for blends, after melting: those results may be related to other phase transitions, e.g. liquid-liquid phase separation. All things considered, the apparatus can be used to get reliable data on phase transitions in polymeric systems.