The physical ageing of polymethylmethacrylate is investigated using a combination of positron annihilation, dielectric relaxation and dynamic mechanical thermal analysis. Correlations are observed between the three types of observation; however, no simple theory can be seen to fit all of the data. The dielectric data indicates that 'thermorheologically simplicity' is probably not a valid assumption for this system. The data also suggest that the free volume distribution probably changes during ageing; however, it was not probably to quantify these changes from the current positron annihilation study. Comparison of the experimental data from these various techniques clearly indicates that processes occurring at a molecular level are complex and not simply the result of one single type of conformational rearrangement and have an apparent activation energy comparable to that of the glass-rubber transition. (C) 1997 Elsevier Science Ltd.