The structural origins of ion recognition by electrochemically addressable poly[Ni(salen)] thin films are explored using in situ X-ray spectroscopy. XANES and EXAFS provided the local environment (nearest and next-nearest neighbours) around the Ni atom and solution-derived Ba2+ bound to the film. The Ni is covalently bound to two N and two O donors in square planar geometry, irrespective of film redox state and the presence (or absence) of bound Ba2+. The role of the Ni is purely structural; dramatic changes in i-E response accompanying Ba2+ uptake are assigned to the delocalised poly(salen) polymer spine. Ba2+ is trapped in a pseudo-crown formed by two methoxy O donors and two O donors shared with the Ni atom. The Ba2+ EXAFS signal from thick films (10 mu m) is significantly below that anticipated from electrochemical observations on thin films (<100 nm). Since EXAFS and XANES integrate populations over the entire film, this suggests that slow transport restricts Ba2+ access to the outer region of the film; surface sensitive XPS data confirm this. Combination of spectroscopic and electrochemical data suggest that, for exposure times of ca. 10(3) s, only sites in the outer ca. 1 mu m of the film are occupied; the implied diffusion coefficient of 10(-11) cm(2) s(-1) is consistent with a relatively compact solvated film. (C) 2007 Elsevier B.V. All rights reserved.