Structural relaxation of poly(methyl methacrylate) has been studied by differential scanning calorimetric (d.s.c.) and dynamic mechanical techniques, and the effect of temperature of ageing has been investigated. Isothermal ageings at temperatures between T-g-30 K and T-g-100 K lead to d.s.c. pre-peaks and a diminution of the mechanical loss tangent. The beta relaxation is not affected by ageing and the cc relaxation exhibits a low-temperature tail within the temperature domain of the beta relaxation. A physical model based upon the concept of annihilation of quasi-point defects through diffusion is used to analyse the results. Diffusion is thought to occur by means of hierarchically correlated movements characterized by a correlation factor b. Simulation of the calorimetric data is satisfactory and comparison with the dynamic mechanical results underlines a specific mechanism for ageing temperatures far below T-g.