Fourier transform infrared (FTIR) and near-infrared (NIR) spectroscopy have been used to characterise the ageing of cellulosic paper, with the long term aim of developing a technique to assess the condition of paper insulation in electrical transformers. There are differences in the spectra of new and aged materials from different sources, which provide the basis of a "fingerprint" method to categorise papers into different "families". Measurements of the rate of development of a carbonyl band during ageing in air give an activation energy of 98 kJ/mol for cellulose oxidation. Applying chemometric methods to NIR spectra of aged cellulose gives a correlation of 0.994 between the spectra and time of ageing, with an error of prediction of 95 h for samples up to 3000 h of ageing. (C) 2001 Elsevier Science Ltd. All rights reserved.