Local structural order and temperature-dependent structural variation have been studied in the molecular-based layer ferrimagnet (eta-C4H9)(4)N (FeFeIII)-Fe-II (C2O4)(3) by EXAFS and high resolution X-ray powder diffraction. The EXAFS spectra measured at the Fe K-edge are successfully modelled by successive O, C, O and metal shells, showing that even when there is extensive structural disorder due to stacking faults, the local structural order in this class of ferrimagnets is fully retained. In this salt, which shows remarkable negative magnetisation at low temperature (Neel class Q), the EXAFS Debye-Waller factor has a discontinuity at 40 K, corresponding to one found in the magnetisation. At the same temperature there is also a change in the expansion of the lattice as evidenced by the high resolution X-ray powder diffraction. (c) 2004 Elsevier Ltd. All rights reserved.