Changes in composts of sewage sludges and green wastes were analysed by near infrared reflectance spectroscopy (NIRS) and chemical analysis with 426 samples representative of six stages of composting: 8, 20, 35, 75, 135 and 180 days. Maturity of compost was assessed through changes in C:N ratio. Results of spectroscopic properties (200 wavelengths) were studied with several multivariate analyses. First, a descriptive approach revealed compost changes with time of maturation. Then, a constrained ordination (RDA with permutation tests) demonstrated a significant effect of three factors of sampling: stage of composting, depth and position in windrows. Precise calibration models between spectral data, the C, N, C:N values and composting time were build using partial least square regression (r(2) > 0.95). Together, these results show the efficiency of NIRS to predict chemical changes and the stage of transformation of organic matter during the composting process. (C) 2007 Elsevier Ltd. All rights reserved.