Blends of poly(ether ether ketone) (PEEK) and poly(ether imide) (PEI) were examined for their miscibility at the nanoscopic level using positron annihilation lifetime spectroscopy (PALS) and modulated differential scanning calorimetry (MDSC). The free volume results obtained from PALS reveal that PEEK and PEI exhibit high degree of miscibility showing interlamellar segregation when the PEI content in the blend is =50%. Although the average free volume sizes of PEEK and PEI are nearly same, a clear distinction could be made from their size distribution. The free volume of PEI is found to be narrow as compared with PEEK. Also, herein, we provide the first evidence of the influence of interlamellar segregation on the free volume distribution in a polymer blend. The crystalline structure of the blend was studied by X-ray scattering and the surface morphology was examined by scanning electron microscopy (SEM). The MDSC results indicate the presence of a possible rigid amorphous fraction (RAF) and corroborate the positron lifetime and X-ray scattering results. (C) 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012