Conformational changes are observed during sub-T-g annealing of solution-cast films of BPA-polycarbonate by following changes in the conformationally sensitive IR aromatic breathing band at 1600 cm(-1). The rate of conformational relaxation is compared with published data on enthalpy and volume relaxation on the same polymer. The relaxation rates and timescales converge in the immediate vicinity of T-g, but diverge at temperatures below T-g - 15 K and conformational energy only accounts for a very small fraction of observed enthalpy relaxation. The results suggest that a single process is active in volume and conformational relaxation and that at least two processes contribute to enthalpy relaxation. (C) 2001 Elsevier Science Ltd. All rights reserved.