The regulating efficiency of 14 dialkyl xanthogen disulfides in emulsion copolymerisation of butadiene with styrene a t +5OC. with the use of a diisopropylbenzene hydroperoxide-complexed ferrous iron-sodium formaldehyde sulfoxylate redox system in the presence of sodium soap of disproportionated rosin as an emulsifier, waa evaluated. The apparent chain transfer constants of n-alkyl and isoalkyl derivatives decreased logarithmically with increasing length of the alkyl group, which see1114 to be related to the analogous dependency of their solubilities in water. The dialkyl xanthogen disulfides, especially the lower homologs, acted as retarders of polymerization. The retardation did not affect their regulation efficiency. On the basis of values of the apparent chain transfer constants it is possibld to predict the molecular weight of polymers, except for the region where the regulation is poor and where the deteriorative influence of termination and crosslinking reactions takes place. The diisopropyl xanthogen disulfide has been selected as the most convenient of the compounds studied for molecular weight regulation of emulsion copolymerization of butadiene with styrene in the system mentioned. 1881 a Data of Vhclavek.2 c Dat,a of gkrabal and Rosik." d Data of Kamen6r.6 Sodium salt of dibutylnaphthalenesulfonic acid.