Storms are responsible for up to ~50 % of total annual rainfall on tropical islands and result in rapid increases in discharge from rivers. Storm events are, however, notoriously under-sampled and their effects on weathering rates and processes are poorly constrained. To address this, we have undertaken high-frequency sampling of Quiock Creek catchment, a Critical Zone Observatory located in Guadeloupe, over a period of 21 days, encompassing several storm events. Chemical and isotopic (Li and Mg) analyses of different critical zone reservoirs (throughfall, soil pore water, groundwater and river water) were used to assess the interactions between rock, water and secondary minerals. The Li concentrations and δ 7 Li values of these different reservoirs range from 14 to 95 nmol/kg and 1.8 to 16.8‰, respectively. After several rain events, the average δ 7 Li value (13.3‰) of soil solutions from the lower part of the soil profile (> ~150 cm below the surface) was unchanged, whereas in the upper part of the profile δ 7 Li values increased by ~2-4‰ due to increased contribution from throughfall. By contrast, the δ 26 Mg value of soil waters in the upper part of the soil profile were not significantly affected by the rain events with an average value of-0.90‰. The δ 26 Mg values of the different fluid reservoirs were generally close to the value of throughfall (~-0.90‰), but higher δ 26 Mg values (up to-0.58‰) were measured in the deeper parts 2 of the soil profile, whereas groundwaters that have a long residence time had lower δ 26 Mg values (down to-1.48‰). These higher and lower values are attributed to, respectively, adsorption/desorption of light Mg isotopes on/from the surface of clay minerals. The δ Li value of the river waters was ~9.3‰, with a Li concentration of µmol/kg, but during a storm these values decreased to, respectively, 7.8‰ and 40 µmol/kg. This change in δ 7 Li is consistent with an increased contribution of Li from the soil solution. Thus, even in highly weathered catchments, changes in hydrological conditions can have a significant impact on weathering processes and therefore the composition of river waters delivered to the ocean.