In situ NMR and extended X-ray absorption fine structure (EXAFS) spectroscopy have been used to characterise a highly active propene dimerisation catalyst, prepared by the low-temperature addition of AlEt₃ to [Ni(η³-C₃H₅)(PPh₃)Br]. The molecular structure of [Ni(η³-C₃H₅)(PPh₃)Br] has been determined by X-ray crystallography and variable-temperature NMR spectroscopy. Crystal data: monoclinic, space group P2₁/_n(no. 14), a= 7.951(1), b= 14.954(1), c= 16.096(3)Å, β= 96.05(1)°, Z= 4, R= 0.045. It has previously been assumed that aluminium interacts with nickel in such catalyst systems via halide-bridging ligands in order to decrease the charge on the nickel, thereby facilitating the co-ordination of electron donors to the nickel centre. EXAFS analysis provided direct structural evidence for a Ni ⋯ Al interaction in solution, but questioned the nature of the bridging ligand: the nickel K-edge EXAFS data of a solution of [Ni(η³-C₃H₅)(PPh₃)Br], AlEt₃(Ni:Al = 1 : 5) and propene in toluene at –60 °C, showed that the first co-ordination sphere around nickel comprised 3.9 carbons at 1.93 Å and 1.0 phosphorus at 2.19 Å, with a more distant aluminium being present at 3.21 Å Halide loss from the nickel centre is further confirmed by the bromine K-edge EXAFS data These results implied Ni ⋯ Al interactions via alkyl bridges in the predominant solution species during catalysis. Phosphorus-31 NMR spectroscopy at –60 °C of the activated catalyst however showed at least four different species present. Halide loss from the nickel co-ordination sphere was also observed for the reaction of AlBr₃ with [{Ni(η³-C₃H₅)Br}₂] in toluene. The nickel K-edge EXAFS was best fitted by 9.0 carbons at 2.01 Å, providing strong evidence for the existence of [Ni(η³-C₃H₅)(η⁶-C₆H₅Me)]⁺-AlBr₄^–.