The detailed molecular interactions occurring during thermal degradation of PVC polymer formulations containing MoO3 additives are investigated using laser microprobe techniques coupled with mass analysis of the volatile pyrolysis products. Comparison with Sb2O3–PVC compounds indicate that the additive effects exhibited by MoO3 are fundamentally different from those observed for Sb2O3. Thermal decomposition of MoO3–PVC is characterized by (1) catalyzed dehydrochlorination of PVC at a lower temperature and increased rate; (2) marked reduction in evolution of benzene, the major fuel species from PVC; and (3) decreased evolution of volatile hydrocarbon species from the polymer plasticizer component. Vapor-phase interactions involving volatile molybdenum species are found to be unimportant. The experimental data indicate that condensed-phase mechanisms and heterogeneous reactions involving MoO3(s) control polymer decomposition processes. Molecular level details of these reactions are presented and their implications to polymer flame retardance and smoke suppression discussed.