Isothermal physical ageing below the glass-transition temperature (T-g) of a high-T-g thermosetting difunctional epoxy/tetrafunctional aromatic amine system was investigated at different ageing temperatures (T-a) and chemical conversions (monitored by the T-g) using the torsional braid analysis freely oscillating torsion pendulum technique. In the absence of chemical reaction during an isothermal ageing process, the rate of isothermal physical ageing passes through a minimum with increasing conversion. The minimum is related to the minimum in mechanical loss between the secondary relaxation in the glassy state (T-beta) and the glass-transition relaxation (T-g) (the temperatures of both of which increase with increasing conversion). If isothermal ageing rates for all conversions (beyond gelation) would have been measured directly from temperatures low T-beta to above T-g it is concluded that two maxima in isothermal ageing rate would have been observed corresponding to the two relaxation processes. There exists a superposition in isothermal ageing rate versus T-g - T-a [by shifting horizontally (and vertically)], which implies that the ageing rate is independent of the details of the changing chemical structure attributed to cure. Controlling mechanisms during physical ageing are segmental mobility associated with the T-g region and more localized motion associated with the glassy-state relaxation T-beta. (C) 2003 Wiley Periodicals, Inc.