The instability and poor reproducibility of polymer inclusion membranes (PIMs) containing Cyanex 272 (di(2,4,4-trimethylpentyl)phosphinic acid, HCyanex) for the extraction of cobalt(II) have been investigated using chemical and spectroscopic means. The differing transport properties of the cobalt: Cyanex adducts, and the formation of deep blue oils exuded from the surface of the PIM under particular extraction conditions are explained in terms of oligomerisation of the cobalt centres with the Cyanex anion acting as a bridging ligand, {Co:Cyanex(2)}(n). Oligomers present within the polymer matrix are not available for back-extraction and if formed on the surface of the membrane are unable to diffuse into the membrane. The X-ray absorption spectra of cobalt(II)-loaded PIMs and of the {Co:Cyanex(2)}(n), oils confirm tetrahedral coordination geometry about the cobalt(II) centre. Analysis of the X-ray absorption fine structure AFS) of the cobalt(II)-loaded Cyanex PIMs and {Co:Cyanex(2)}(n), oils both give a local structure with unidentate phosphinate ligands bridging cobalt centres in an extended structure closely related to polymeric cobalt(II) phosphinate forms previously characterised by X-ray crystallography. These results show clearly the impact of oligomerisation on the migration properties of metal:extractant adducts within PIMs which can be responsible for substantial differences in the performance of extractants in solvent extraction and PIM applications. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.