The behavior of terazosine hydrochloride (TR), representative of the selective α₁-adrenergic blockers in river water under the influence of sunlight, was tested. Details of the photodegradation process were obtained using principal component analysis (PCA) and multivariate curve resolution alternating least squares (MCRALS) chemometric methods and confirmed by HPLC analysis. Additionally, the impact of some factors important for efficient waste treatment were studied. For this purpose a stability of terazosine aqueous solutions under the influence of UV, UV+H₂O₂, Fenton, and photo-Fenton processes and sunlight was studied. The presence of H₂O₂ and Fe (II) ions increases the rate of transformation. The process of direct photolysis and H₂O₂-assisted photoreaction showed first order kinetics, while dark and photo-Fenton reactions exhibited second- order kinetics.