The reflectance spectra of organic matter in the VIS-NIR-SWIR regions (400-2500 nm) were investigated with regard to possible changes that might occur during a biological decomposition process. Two different groups of organic matter were used in this study: a grape marc (CGM) and a separated cattle manure (CSM) that simulated pure organic matter endmembers in soils. Exposing the two materials for different decomposition durations (0-378 days) visually yielded color sequences as the compost aged. Significant changes in the reflectance spectra of both materials were also observed during the composting period, which provided parameters for controlling the composting process. The slopes in the VIS-NIR region were found to be basic parameters for monitoring changes and were found to be highly correlated with other chemical parameters often used for assessing organic matter conditions in the field (such as the C/N ratio). It was found that during the initial composting stage (0-60 days) the slope parameters were strongly affected by the decomposition activity and, hence, errors in the assessment of organic matter content of soils using slope (or band ratio) parameters are likely. Careful observation of the major spectral features reveals that the reflectance spectrum in the VIS-NIR-SWIR region is a very sensitive tool for monitoring slight changes. Application of the near-infrared analysis (NIRA) pathways revealed that OH and C-H groups combined with hygroscopic water, starch, cellulose, and lignin are the components having the highest correlations with composting time within the conditions used. Because of the small number of samples in each testing group a complete NIRA employing validation tests could not be carried out. We concluded that the reflectance spectrum in the VIS-NIR-SWIR region is a promising tool for monitoring the composting process and that the composting process may provide invaluable spectral information about soil organic matter during its biochemical degradation.