In recent years, the increase in the production of synthetic organic compounds and their constant release into the environment, especially in reference to pharmaceuticals, compounds on which this thesis has focused, has meant that these substances of origin generally anthropogenic have been accumulating in biological ecosystems in an important, extensive, and persistent way, especially in the aquatic environment. For this reason, it is necessary to use environmental purification processes to eliminate substances that are harmful to health and the environment. However, the purification technologies used today do not yet allow these substances to be completely eliminated in a quantitative way. In addition, the potentially harmful effects of these substances on the environment are not only due to the original compounds, but in many cases also to their degradation products and metabolites. Consequently, it is important to know the fate and transformation that organic pollutants undergo once they enter the ecosystem and reach the aquatic environment. We must therefore keep in mind that water is an essential resource for life that requires the best possible sanitary quality for its consumption. Dissolved organic matter (DOM), present in both surface and groundwater, and which has a significant effect on the biochemical processes of aquatic systems, often acts as an indicator of the level of quality and health of water. Therefore, when optimizing the purification processes carried out, it is also very important to know and act on the concentration and characteristics of the DOM present in it. For this reason, the following studies have been performed in this Thesis: On the one hand, to better understand the process of photodegradation of drugs, the degradation process of two of them has been studied. In a first study, different samples of a cytostatic drug (tamoxifen) were subjected to a controlled source of UV radiation in the laboratory, which simulates solar radiation (suntest), using two different irradiation powers. At the same time, its photodegradation is monitored using UV-Visible spectrophotometric measurements and multidimensional fluorescence (excitation-emission matrices, EEM). Then, different sample aliquots along the degradation are analyzed by LC-DAD, LC-MS and / or LC-FLD. These analyzes, which generate a very high amount of data, are merged and treated simultaneously using advanced chemometric procedures (especially with multivariate curve resolution alternating least squares method (MCR-ALS)), which describe the process and the degradation reaction, solve the different transformation products (TPs), identify them and evaluate the kinetics associated with the process studied. As a result of this study, 3 TPs and 1 isomer of tamoxifen were characterized. A second study with the same aim was carried out on the antibiotic sulfamethoxazole, present in rivers and effluents from all over Europe in remarkable concentrations. Additionally, a study of the acid-base properties of this substance was conducted to see how pH can affect the speciation of this compound. Similarly, to know whether the photodegradation of the drug is influenced by this parameter, the degradation process was studied using sample solutions at different pH. As a result of the chemometric analysis of the data from this second study, a conformational isomer, an unknown photoproduct, and 4 TPs were characterized. On the other hand, 3 different forms of the antibiotic were detected in the corresponding acid-base titration. On the other hand, to know the quality of the surface water of the rivers, different samples of surface water from the Llobregat river basin have been analyzed and characterized by multidimensional fluorescence (EEM). In this study, the monitoring of water quality from the combination of the fluorometric measurements of EEM and its flexible chemometric modeling, also through the MCR-ALS, has made it possible to obtain information on the nature of the fractions of the DOM (humic, fulvic, protein origin, etc.) present in the Llobregat river and its geographical distribution along the basin in different environmental sampling campaigns. Therefore, given that there is an urgent need today to develop new multivariate data processing tools, such as those obtained in this Thesis in the different cases of environmental interest studied, the presented report aims to contribute to solve this bottleneck from the use of chemometrics, and especially the MCR-ALS method, which can play a very important role in solving many of the problems associated with data analysis in different areas of chemical and environmental application.