AbstractPrompted by commercial significance and academic interest, extensive model compound andin‐situstudies of PVC degradation/stabilization appear in the literature. This paper presents the results of somein‐situstudies of the chemical reaction pathways undergone by organotin thioglycollate esters during PVC stabilization. Infrared spectroscopy indicates that in the case of diocyltin bis(isooctylthioglycollate), not only is the mercaptoester/chloride generated, but also the corresponding thioglycollate (Oct2Sn TG). The extent to which this species is observed is dictated by the processing technique. An attempt is made to correlate the chemical reaction pathway of the organotin compounds with the changes (unsaturation) within the polymer. The possible relevance of these observations to the phenomenon of mono/di synergism is also discussed. While these studies extend our knowledge of the organotin stabilization of PVC, they also show that our understanding is far from compl