The phase diagram of an isotactic polypropylene/poly(ethylene-octene) copolymer (iPP/PEOc) blend system was investigated using phase contrast optical microscopy, laser light scattering and differential scanning calorimetry (DSC). The sample goes through immiscible (opaque) region to transparent region (seemingly miscible) and back to immiscible (opaque) again as temperature increases through 300 degrees C region. But it turns out that this is not a real one phase region. It is caused by a temperature dependent inversion of refractive indices between the two component polymers, which can be easily misinterpreted as a miscible region between an upper critical solution temperature (UCST) state and a lower critical solution temperature (LCST) state. With a proper interpretation and analysis of this refractive index inversion, the UCST phase diagram of this iPP/PEOc blend system has been obtained. (C) 2010 Elsevier Ltd. All rights reserved.