AbstractA wide range of compounds of the Group VIII metals has been investigated as possible catalysts for the polymerisation of butadiene and isoprene in the aqueous phase. The compounds studied included the fluorides, chlorides, bromides, iodides, nitrates and sulphates, and also certain chelate complexes of the Group VIII metals. Substantial amounts of polybutadiene were obtained using rhodium chloride, rhodium bromide, iridium chloride, platinous iodide, and iron (III) acetylacetonate. The first three gave stereoregular (hightrans‐1,4‐) polybutadiene; the last two gave mixed microstructures. With isoprene, significant polymerisation was observed with iridium chloride, platinous iodide, and iron (III) acetylacetonate. In each case, the product was non‐stereoregular.A more detailed study of the rhodium chloride‐butadiene system has indicated that the locus of the polymerisation is the aqueous phase, probably the surfactant micelles. A mechanism for the polymerisation has been proposed in the light of these results. This mechanism suggests certain predictions regarding the effects of chloride ions and of isoprene upon the polymerisation rate. These predictions have been tested experimentally, as also has the prediction that the polymer should contain chemically combined sur