A long-term natural ageing programme was started in 1958 when 19 rubber compounds were exposed at 3 locations. The final sets of test pieces were withdrawn in 1998 giving a total of 40 years of natural ageing. This report details the results of accelerated heat ageing studies undertaken on re-mixed samples of those materials studied for the natural ageing study and on the 20 new compounds chosen to represent polymers not available in 1958 and to reflect changes in compounding practice.