Chemical weathering of silicates, in particular mafic rocks, is a major driver of global climate over geological time scales via uptake of atmospheric CO 2, but the magnitude of atmospheric CO2 consumption by this process is still debated. Chemical weathering of mafic rocks is accompanied by a high consumption of atmospheric CO 2. In spite of mafic watersheds often being small, weathering of mafic rocks is therefore very important for the global carbon cycle. A strong positive correlation between chemical and physical weathering exists, as well as between chemical weathering and temperature. One would therefore expect to find the highest chemical weathering rates and atmospheric CO2 consumption in tectonically active, hot and humid locations. I investigate the magnitude of atmospheric CO 2 uptake due to silicate weathering in two different tropical settings, Hawai'i and the Philippines. These two locations have broadly similar climate but vary significantly in their lithology and tectonic style. Rivers and streams were sampled in both locations, as well as hot springs in the Philippines and fresh groundwater in Hawai'i. The water was analyzed for dissolved chemicals and fluxes of atmospheric carbon and other elements from the watersheds were calculated using available hydrological data from the University of Diliman (the Philippines) and the USGS (Hawai'i). In Hawai'i, a runoff kriging model, which was used to calculate discharge in unmonitored watersheds, was created. The results indicate that atmospheric CO 2-fluxes in the Philippines are among the highest yet reported from any silicate environment in the world, with an average of 3.58 ± 0.23 x 106 mol/km2/yr from volcanic areas. Atmospheric CO2-fluxes from streams in Hawai'i are close to the global average for basaltic regions. Groundwater discharge is an important part of the hydrological budget in Hawai'i and including chemical fluxes via groundwater increases the total chemical flux from the islands by 30-95%. Importantly, my results imply that groundwater is an important pathway for delivery of dissolved chemicals to the ocean at all stages in the life of a volcanic island.