Four new β-triketimine cobalt complexes [(LCo-μ-Br)2][BArF]2 where L = N,N′,N′′-triaryl-β-diketimine were synthesised and then characterized by single-crystal X-ray diffraction, MALDI-MS and elemental analysis. In combination with three previously reported, they provided a series of seven subtly different complexes which were screened for activity in the catalysis of isoprene polymerization: the structure of the ligand (L) had strong effects on activity and stereoselectivity. The produced polyisoprene contained a mixture of cis-1,4, trans-1,4 and 3,4-enchained monomers; 4,1-errors followed 3,4 errors. The highest percentage of cis-1,4 content (80%) was obtained with complex 5 where two electron-withdrawing fluorine substituents occupied the 2- and 6-positions of two of the imino-aryl rings. This polymer was obtained in 98.5% conversion. Both cis-1,4 content and activity decreased when electron donor substituents were present on the aryl rings, so that complex 1, with three methyl substituents in positions 2, 4 and 6 gave the lowest cis-1,4 content (73.6%), with a conversion of only 58.5%, under identical conditions. The temperature of the polymerization, the type of aluminium co-catalyst used, and the Al/Co ratio strongly affected the activity and microstructure of the polyisoprene produced. Ethylaluminiumsesquichloride was the most active of a range of organoaluminiums screened. A kinetic study using complex 6 as catalyst demonstrated that the polymerization was first-order in monomer, and that approximately 12% of cobalt formed active centers. The combination of high molecular weight (>105) with moderately high activity at conveniently accessible temperatures to give predominantly cis-1,4-polyisoprene but with 3,4 units to promote efficient crosslinking is potentially attractive, and has not previously been attained with 3d elements.