In situ studies have been performed on a series of homogeneous nickel alkene oligomerisation catalysts using the techniques of Extended X-ray Fine Structure (EXAFS) and NMR, with the characterisation of catalytic species being supported by the study of standard compounds. For the ethene oligomerisation catalyst formed by addition of Ph₂PCH₂C(CF₃)₂OH to Ni(cod)₂ in toluene, both ¹H and ³¹P NMR show the nickel hydride observed before the addition of alkene to be [(Ph₂PCH₂C(CF₃)₂O)NiH(Ph₂PCH₂C(CF₃)₂OH)]. On addition of ethene similar phosophorus coordination is seen by ³¹P NMR, but the hydride is lost, indicating formation of [(Ph₂PCH₂C(CF₃)₂)O)NiR(Ph₂PCH₂C(CF₃)₂OH)] (R= alkyl). The presence of such alkyl species was confirmed by Ni K-edge EXAFS, with an observed increase in the coordination number of light backscatterers (carbon/oxygen) at ca. 2.0AA, on addition of alkene to the catalyst solution. The in situ ³¹P NMR of the propene dimerisation catalyst formed when (allyl)NiBrPPh₃ is activated by AlEt₃, in toluene, shows the phosphine remains coordinated to the nickel centre but in the form of at least five different nickel species. For the EXAFS of this system during catalysis, with a 1:5 excess of AlEt₃ and propene present, the Ni and Br K-edge results both show loss of bromine from the nickel coordination sphere, with the Br K-edge indicating the halide has transferred to the aluminium centre. The number of carbon backscatterers observed by the Ni K-edge EXAFS increases on activation, inferring that alkylation of the metal centre has occurred. Furthermore the formation of bridged Ni-C-Al species is suggested by a Ni-Al distance of ca. 3.2AA. The Ni and Br K-edge EXAFS of the (allyl)NiBrPPh₃/AlEt₃ catalyst in the absence of propene gave conflicting results, indicating the activation process is a finely balanced equilibrium.