the volume and enthalpy relaxation in a-PMMA subjected to temperature jumps in tg region has been analysed. The measured H and V data were compared with respect to aging time and proportionality between them as a slope of (∂H/∂V)T dependencies has been found. According to previous works the slope was identified as an apparent bulk modulus, Ka. This method is applied to aging following temperature up-jumps after consolidation periods of varying lengths. the main
finding is a marked increase of Ka with consolidation time, approaching a limiting value in an asymptotic fashion.