AbstractQuantitative119Sn‐NMR spectroscopy is a valuable tool to identify and determine quantitatively all organotin species which are formed in PVC during processing or a thermal degradation test by reaction with the polymer or by subsequent ligand exchange reactions between different organotin compounds. By this method conversion rates of monoalkyltin and dialkyltin isooctylthioglycolates in PVC have been determined as a function of the duration of thermal treatment. It is shown that organotin stabilisers can also react with lubricants and other additives to compounds like R2Sn(SR)(OCOR) during the processing of PVC, thus influencing the efficiency of the stabilising actio