The thermal stability of papermaking pulps is considered with particular reference to the criteria for establishing thermal stability indices. The difference between several indices is examined. The residual weight after pyrolysis is shown to be a useful guide to low temperature thermal stability. A new stability index is formulated and used to rank papermaking pulps. An index independent of the heating rate allows stabilities of materials to be compared even though they were tested initially at different heating rates. Thermal stability rankings according to activation energies did not correlate well with rankings obtained by empirical indices of stability or by the rate of loss of physical properties with accelerated aging. A much closer relationship exists between rankings based on changes in physical properties and empirical indices derived from the thermogravimetric profiles. See also AATA 15-389.