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We report the changes of atactic polystyrene (a‐PS) upon annealing below the glass transition based on X‐ray scattering, volume, and differential scanning calorimetry measurements of samples in the quenched and annealed states. Upon relaxation, an increase of the main diffuse diffraction peak and a decrease of the polymerization peak, typical for a‐PS, are proportional to the enthalpy changes. The relaxation of enthalpy corresponds to a change of the fictive temperature by 6.1 K for annealing at 80°C for 24 h. The changes of the fictive temperature derived from the X‐ray structure factor are between 5.4 and 6.8 K, depending on the range of scattering vector. The relaxation signal has a zero crossing at low scattering angles. The volume of the samples, measured at ambient conditions, did not change upon relaxation. We attribute these observations to localized relaxation processes that involve a small number of phenyl groups. [ABSTRACT FROM AUTHOR]