Despite the significant contribution of volcanic regions to the global flux of solutes to the ocean and hence, to the consumption of atmospheric CO2, uncertainties remain regarding the variables which control riverine weathering fluxes in these regions. Active hydrothermal systems and the weathering of alteration deposits from extinct hydrothermal systems can act as a source of acidity for weathering reactions in addition to atmospheric CO2. Moreover, the influence of seasonal runoff variability in volcanic regions on the relative contribution from hydrothermal and meteoric processes to riverine weathering fluxes remains poorly constrained. These uncertainties prevent accurate estimates of atmospheric CO2 consumption from these regions. The aim of this PhD thesis is to better understand the factors controlling weathering fluxes in rivers draining hydrothermally-active volcanic regions by combining riverine element concentrations and two geochemical tracers, the germanium to silicon ratios (Ge/Si) and the stable silicon isotopes (δ30Si). The first step was to develop a method for the reliable determination of δ30Si values of anion- and cation-rich thermal springs and river waters affected by hydrothermal discharges. This PhD study demonstrates that: (i) the intensity of water-rock interaction is a major control on the variability of Ge/Si ratios and δ30Si in thermal waters, evolving from immature to mature thermal waters, (ii) the riverine Ge/Si ratios and δ30Si values can be used as indicators of the source of acidity for weathering in volcanic terranes featuring hydrothermal manifestations, (iii) the riverine Ge/Si ratios can serve to quantitatively assess the hydrothermal contribution in a river draining a hyper-acid hydrothermal system, (iv) the seasonal change in Ge/Si ratios and δ30Si values between base flow and high discharge periods in rivers draining hydrothermally-active volcanic regions highlights short pulses of seasonal meteoric weathering contribution which should be accounted for as they directly contribute to atmospheric CO2 consumption. Overall, this PhD study provides novel insights to quantify the sources of acidity controlling riverine weathering fluxes in hydrothermally-active volcanic terranes, thereby improving our ability to estimate the atmospheric CO2 consumption associated to weathering in volcanic regions.