This study made use of the technique of positron annihilation spectroscopy (PALS) to examine the effect of a compatibilizer additive on the compatibility in a blend membrane of polycarbonate (PC) and poly(2-hydroxyethyl methacrylate) (PHEMA) and on the pervaporation (PV) performance of the compatibilized blend membrane. Scanning electron microscopy (SEM) showed that a blend membrane consisting of only PC and PHEMA apparently had some extent of incompatibility between the component polymers. An attempt to improve the compatibility was done by adding a graft copolymer (PC-g-PHEMA) compatibilizer. Results indicated that the addition of this compatibilizer could increase the extent of compatibility in the blend as shown by the SEM micrographs, strengthen thermal stability based on the thermal gravimetric analysis (TGA) thermograms, and effectively improve the pervaporation performance of the compatibilized blend membranes. The improvement in the compatibility, associated with better interfacial adhesion, was correlated with the microstructural properties obtained from PALS, such as ortho-positronium (o-Ps) lifetime and intensity, free volume size and distribution, and fractional free volume. (C) 2011 Elsevier B.V. All rights reserved.