Abstract : Natural organic matter (NOM) is a complex mixture of molecules formed from decomposition of the dead organic matter of animals and plants. A fraction, refered to as humic substances, absorb solar light and exhibits photosensitizing properties. The objective of this thesis was to investigate the photodegrading properties of a variety of humic substances. In particular, we tried to find correlations between these properties and spectral characteristics (absorbance, fluorescence). The goal was to identify the absorbing entities responsible for the photosensitizing properties. Many samples were studied: humic acids extracted from soils, fractions of these humic acids obtained by ultrafiltration, water extracts of soils, humic-like acids extracted from compost, water extracts of compost. For all, spectral and photosensitizing properties were investigated simultaneously. Two molecular probes, 2,4,6-trimethylphenol and furfuryl alcohol were used. The former is oxidized by humic triplet excited states through electron transfer and the latter is photooxygenated by singlet oxygen produced by electron transfer between humic triplet excited states and ground state oxygen. Active ingredients of phytosanitary formations were tested too. The following important results were obtained. The components that are fluorescent above 500 nm are located in the macromolecules of molecular weight lower than 30 kDa. The emission band is large and can be splitted in two. The one, with an emission maximum at 515 nm, corresponds to the fraction of molecular weight less than 3 kDa; the other, with an emission maximum at 545 nm, corresponds to the fraction of molecular weight higher than 3 kDa. The production of singlet oxygen is positively correlated with the intensity of emission between 500 and 550 nm. This correlation was obtained for many samples and for very different samples. This means that photosensitizing components are fluorescents at long wavelength. This results is an important step in the understanding of the photochemical behaviour of humic substances and should be of help for the characterization of photosensitizing chromophores.