The effect of demineralisation using hydrofluoric acid (HF) treatment on the elemental, isotopic and chemical composition of organic matter with increasing degree of humification was examined using four horizons of a forest humus profile. The aim was to determine whether HF treatment alters pristine organic matter or not. The conceptual approach included elemental and isotopic analyses of the L, Of, Oh and Ah horizons of a Dystric Cambisol under forest before and after treatment with 2% and 10% HF. The chemical composition of the bulk organic matter was analysed using 13C CPMAS NMR spectroscopy, Fourier transformed infrared (IR) spectroscopy and pyrolysis coupled to gas chromatography and mass spectrometry (GC/MS). The lignin and non-cellulosic sugar components were characterised by wet chemical analysis. Carbon loss after HF treatment was between 7% and 23% of initial C and nitrogen loss between 14% and 27% of initial N. The HF concentration (2% and 10%) influenced the C and N concentration of the HF residue but in most cases not the C and N loss. No change in the isotopic signatures (δ13C and δ15N) were apparent. The bulk chemical composition as seen by 13C CPMAS NMR spectroscopy did not change after HF treatment. (FTIR) spectra of organic horizons were also not affected but analysis of organic matter of mineral rich Oh and Ah horizons became possible only after the treatment. These two methods were not sensitive enough to record changes in OM composition induced by the HF treatment. Pyrolysis GC/MS showed, in agreement with the wet chemical analysis of non-cellulosic sugars, that some change occurred in the quantity and composition of these saccharides. Lignin analysis suggested that some changes occurred in the composition of the molecule.