Several types of additives that contain transition metals can promote the cross-linking of poly(vinyl chloride) (PVC) by a mechanism that apparently involves reductive coupling of the polymer chains. In solid PVC, the cross-linking occurs at 200 degrees C, and model-compound experiments show that it can be ascribed to the preferential reductive coupling of allylic chloride structures when the coupling agent is Cu(O). However, the concurrent coupling of other chloride moieties has not been entirely ruled out. The evidence for reductive coupling consists of rapid gel formation accompanied by, substantial reductions (or minor changes) in the rates of total mass loss (as determined by thermogravimetric analysis), C-C formation (as observed by Fourier transform IR spectroscopy), and HCl evolution (as determined by acid-base titrimetry). Additives that promote the coupling process are sources of a zero-or low-valent metal upon pyrolysis. These additives include a number of transition-metal carbonyls, divalent formates or oxalates of the late transition metals, simple Cu(I) halides, and various complexes of Cu(I) containing phosphites or other ligands. Since the reductive coupling agents tend to have low acidities, they are not expected to promote the cationic cracking of char. Thus they are potentially attractive as replacements for the PVC smoke suppressants that stimulate cross-linking by acting as Lewis acids.