Free volumes in a copolymer consisting of p-hydroxybenzoic acid (HBA) and poly(ethylene terephthalate) (PET) were probed by positron annihilation technique. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured in the temperature range between 30 and 230 degrees C. Above the glass transition temperature (ca. 60 degrees C), the lifetime of ortho-positronium (o-Ps) annihilated in the free volumes and its annihilation probability were found to increase with increasing specimen temperature. These facts were attributed to the increase both in the size of the free volumes and in the concentration of such regions due to rearrangements of molecules. From the observed lifetime of o-Ps, it was found that the size of the free volumes increases from 0.05 nm(3) to 0.1 nm(3) in the temperature range between 30 and 230 degrees C. At solid-mesophase transition temperature (150-220 degrees C), the size of the free volumes monotonously increases, while the increase in the concentration of such regions saturates at 174 degrees C. The clear relationship between the data obtained by the positron annihilation and those obtained by differential scanning calorimetry was confirmed.