The island of Basse-Terre, part of the Lesser Antilles in the western Caribbean, has been affected for centuries by both magmatic and phreatic eruptions from the volcano La Soufrière de Guadeloupe. In particular, the hot and toxic circulating fluids affect the slope stability of the volcano and increase the risks for various landslides. The influence of these hydrothermal fluids on the geotechnical parameters of the material is explained in this thesis. For this purpose, ten samples of in-situ altered material were taken, sealed and later analyzed in a soil laboratory. Six of them came from the immediate environment of the summit (Cratère sud and Gouffre Tarissan), three from the Faille la Ty fault zone south of the peak, and another from a landslide to the southeast of the peak. Sieve and sedimentation analysis, direct shear test, Atterberg limits, pH, grain density, water content, XRF, and XRD were used for classification of the material. The degree of alteration could be determined to range between argillic and advanced argillic, as represented by the alteration minerals kaolinite/halloysite, smectite and opal. Smectite is recognized only in the samples collected around the summit and is no longer represented in the more advanced altered samples. Samples with increasing content of this clay mineral show higher friction angles and cohesion, and thus higher strength. Their plasticity is also increasing. Kaolinite/halloysite-containing material shows higher cohesion while still lowering the friction angle. The higher the amount of kaolinite/halloysite, the higher the shear strength is. Amorphous silica phases, such as opal, have been found in the highly altered zones. The increasing content has a positive effect on the cohesion, a negative one on the friction angle and no consequence on the strength. The higher amount of amorphous silica decreases the plasticity of the material. The progression of alteration shows no influence on the grain density nor the proportion of fine particle size. It is spatially variable, with no clear link to distance from active craters, such that alteration-induced material weaking cannot be identified easily at the surface.