The kinetics of the pyridine-initiated polymerisation of simple 1,3-oxathiolan-2,5-diones (3a-c) in nitrobenzene was studied at various temperatures. Results show that, although the substituents in 4-position have a marked effect on rate of polymerisation, the polymerisation of the three monomers studied is linked by a common mechanism. Thus, the pyridine, nucleophile attacks the carbonyl in 5-position and forms a charge transfer complex which decomposes to form a highly reactive a-thiolactone intermediate 8 together with carbon dioxide and pyridine. Chain addition continues by the direct attack on the thiolactone by the terminal-SH group of traces of parent acid. The reaction of each of the three monomers with pyridine was found to show first order kinetic dependence on both monomer and pyridine, although the initiator (i. e. pyridine) is not consumed. The resultant polymers (molecular weight typically around 20000) are highly crystalline and relatively insoluble in common organic solvents.