Time-dependent structural relaxations in a melt-mixed 38/62 vol% poly(ethylene oxide)/atactic poly(methyl methacrylate) blend were studied using several techniques : differential scanning calorimetry, pressure-volume-temperature analysis, positron annihilation lifetime spectroscopy, dynamic mechanical analysis, and solid-state nuclear magnetic resonance. The internal volume (free volume hole size) and the external volume (specific volume) of the blend are found to decrease with aging time. The time scale of the volume changes is the same, suggesting that internal and external volumes can be calculated from each other. Increasing mobility of poly(ethylene oxide), composition fluctuations, and shifting glass transition temperatures are observed upon aging. Phase separation in terms of spinodal decomposition below an upper critical solution temperature occurs within minutes and results in two amorphous phases of different composition. Subsequent crystallization then causes further structural changes.