Sewage sludge samples representing different stages during waste water and sewage sludge treatment were collected at four Austrian municipal waste water treatment plants. Changes of sludge composition are reflected by a specific infrared spectroscopic pattern. Anaerobically digested sludge was subjected to aeration in lab-scale reactors in order to find out if post-aeration after anaerobic digestion provides enhanced organic matter degradation and stabilization. Spectral data were evaluated by means of multivariate statistics. Similar spectral characteristics of sludge degradation stages were visualized by principal component analysis. The effect of additional aerobic treatment of anaerobically stabilized sludge was revealed by discriminant analysis that distinguishes additionally aerated sludge from all the other degradation stages of sludge because of changes in the spectral pattern by increasing stabilization. Based on partial least squares regression (PLSR) a correlation coefficient of R(2) = 0.91 was found between spectral characteristics and the chemical oxygen demand (COD). (C) 2008 Elsevier Ltd. All rights reserved.