The inherent thermal stability of rigid polyurethane oams is based on their high isocyanate contents that result in cyclic isocyanurate moieties. Other factors that contribute to flame resistance include the diisocyanate structure and the presence of polyisocyanurate carbodiimide and amide modified isocyanurate. Polyester based foams generally perform better than polyether based foams. For polyethers, however, an increase in chain length leads to an increase in the limiting oxygen index. To study the effects of polyol chain length on the thermal degradation of urethane modified polyisocyanurate foams, experiments focused on the behavior of foams that contained equivalent nominal polyether content at approximately 25 percent, but had different molecular masses. Nuclear magnetic resonance, thermogravimetric analysis, and differential scanning calorimetry provided a consistent profile. Thermal degradation proceeded much faster in the samples that contained the lowest molecular mass polyether. 6 refs.