Here we present the first large dataset of dissolved silicon isotopes signatures (δ30Si) in different tropical rivers, including the Amazon and the Congo, the two largest silicon suppliers to the ocean. A one-year-long monthly series is presented for the Congo River, upstream from the Kinshasa/Brazzaville urban zone. Spatial and temporal variations in the Amazon River and its main tributaries have been studied. Both Congo and Amazon convey similar mean δ30Si signatures (close to +0.8 ) to the ocean, in the range of previously published values. The Congo River exhibits limited seasonal variations, with the exception of some large δ30Si variations that seem related to the presence of high biogenic silicon content in the water column. This could be due to diatom activities at the level of the Pool Malebo, a lake-like widening in the lower reaches of the Congo River upstream of Brazzaville. In contrast, other rivers such as the Mekong (Vietnam) and the Tana (Kenya) can exhibit much higher δ30Si values of up to +2 . These differences may be linked to different parameters including higher weathering rate and human impact.