The temperature dependence of free volume in the blend system of commercial PalphaMS(0.5)-co-AN(0.5) (Luran KR 2556)/PMMA(0.95)MA(0.05) (Lucryl G77) has been investigated by means of positron annihilation lifetime spectroscopy (PALS). The PALS measurements show characteristic changes in free volume parameters at temperatures that coincide with the cloud point and spinodal temperatures of the system. Hence, PALS seems to be a powerful method to detect in-situ both binodal (cloud point) and spinodal phase separation temperatures in polymers. The binodal can be detected at a very early stage.