AbstractData presented for a range of vulcanized rubbers prepared under different conditions show that while olefinic unsaturation becomes reduced by vulcanization in the proportion of one double bond for each sulphur atom combined with a C5H‐unit, the original H/C ratio of 8/5 is not altered. The loss of unsaturation is somewhat modified when zinc oxide or certain accelerators are also present. Oxygen has a slight inhibiting effect on vulcanization; hydrogen sulphide and mercaptans markedly catalyse the vulcanization reaction without, apparently, affecting the efficiency of the sulphur cross‐linking reaction.In confirmation of the results of Hull, Olsen, and France1, a small proportion of zinc oxide or a soluble zinc soap promotes a reaction between sulphur and mercapto groups whereby di‐ and poly‐sulphides arc formed with liberation of hydrogen sulphide. The same type of activity is shown by some of the nitrogenous accelerators commonly used in rubber vulcanization. Substantial conversion of mercapto groups into polysulphidc linkages is therefore to be expected when vulcanization is conducted in the presence of these auxiliary sub