The kinetics of chain scission and major changes in functional groups of natural rubber (NR) during ozonolysis in chloroform at ODC have been followed by gel permeation chromatography (GPC) and Fourier transform infrared (FTIR) spectroscopy. Reduction in molecular weight occurs most noticeably in the first minute of ozonolysis to give products with a bimodal distribution of molecular weight and then more gradually. After 20 min of ozonolysis, the number average molecular weight ((M) over bar(n)) falls to less than 900 and the molecular weight distribution becomes close to unimodal again. These findings are interpreted in terms of an initial, rapid, diffusion-controlled process of ozonolysis, during which polymeric ozonides and peroxides (and possibly also hydroperoxides) are formed, followed by a slower second stage during which the ozonides and peroxides are cleaved, possibly by reaction with further ozone, to give oligomers with various oxygenated end groups. There may also be a minor competing crosslinking reaction initiated by radical intermediates from the breakdown of peroxides and hydroperoxides. Examining the products of ozonolysis at various times by FTIR spectroscopy allows changes in the relative concentrations of the oxygenated functional groups, such as carbonyl, ozonide, hydroxyl and/or hydroperoxide, introduced during ozonolysis and accompanying the reductions in molecular weight, to be monitored.