The dealumination of mordenite was carried out at 100 degrees C with nitric and oxalic acid; after calcination, the samples were characterised using various techniques. The bulk composition showed that both acids were very effective in removing aluminium, but that aluminium was extracted at a higher rate by oxalic acid even at low concentrations. Nitrogen adsorption isotherms and scanning electron microscopy (SEM) images showed the formation of a secondary pore system in the zeolite after treatment with nitric acid but not with oxalic acid. NMR analysis revealed that, at the same Si/Al ratio, the mordenite was almost free of extra-framework aluminium (EFAl) after nitric acid extraction, whereas after extraction with oxalic acid half of the aluminium was octahedrally co-ordinated. The Si(0Al) peak in the Si-29 NMR was found to be split when dealumination with nitric acid was carried out due to the presence of less EFAl. Treatment with oxalic acid led to the formation of a larger number of lattice defects.