The temperature dependence of the average free volume hole size and its distribution in blends of poly(methyl methacrylate) and poly(ethylene oxide) were studied by positron annihilation lifetime spectroscopy. Measurements at room temperature, 70 degrees C, and 110 degrees C all indicate that the average ortho-positronium lifetime (tau(3)), and thus the free volume hole size of the blends, shows a positive deviation from additivity of the values of the pure components. Mean ortho-positronium (o-Ps) lifetimes and intensities were calculated with the POSITRONFIT program, while o-Ps lifetime distributions were obtained from evaluations made with a recently introduced program based on Bayes’ theorem and the maximum entropy principle. o-Ps lifetime distributions were successfully calculated for all measured spectra and offer valuable additional information about the structure of the studied blends. At 70 degrees C, the distributions seem to reflect the lifetimes in a blend of binodal composition. Large differences in width of the o-Ps lifetime distributions as a function of blend composition and temperature are observed.