Subglacial chemical weathering plays a key role in global silicate weathering budgets, contributing to the cycling of silicon (Si) in terrestrial and marine systems and the potential drawdown of carbon dioxide from the atmosphere. Here, we use data from two Greenland Ice Sheet (GrIS) catchments to demonstrate how Si isotopes from dissolved and amorphous particulate fractions (delta(DSi)-D-30 and delta(3)(0)ASi respectively) can be used together with major ion data to assess the degree of secondary silicate weathering product formation and redissolution in subglacial environments. We compare a time-series of summer melt seasons from the two study sites, which differ in catchment size (similar to 600 km(2) for Leverett Glacier (LG) and similar to 36 km(2) for Kiattuut Sermiat (KS)). Subglacial waters from LG have elevated Na+ and K+ ions in relation to Ca (2+) and Mg2+ ions, indicating a predominance of silicate weathering, whilst meltwaters from KS are characterised by carbonate weathering (hydrolysis and carbonation) throughout the melt season. Both catchments have mean delta(DSi)-D-30 values substantially lower than average riverine values (KS 0.41 parts per thousand, LG -0.25 parts per thousand, versus a global riverine mean of 1.25 parts per thousand) and display a seasonal decline, which is more pronounced at LG. The delta(30)ASi values (discharge weighted mean values KS -0.44 parts per thousand, LG -0.22 parts per thousand) are lighter than the bedrock (mean values KS -0.18 +/- 0.12 parts per thousand, LG 0.00 +/- 0.07 parts per thousand) in both catchments, indicating a secondary weathering product origin or leaching of lighter isotopes during initial weathering of crushed rock. When used in combination, the major ion and silicon isotope data reveal that the extent of silicate weathering and secondary phase redissolution are more pronounced at LG compared to KS. Contrasting weathering regimes and subglacial hydrology between catchments need to be considered when estimating the delta Si-30 composition of silica exported into polar oceans from the GrIS, with larger catchments likely to produce fluxes of lighter delta Si-30. As larger catchments dominate freshwater export to the ocean, GrIS meltwater is likely to be very light in isotopic composition, and the flux is likely to increase with ice melt as the climate warms. (C) 2019 Elsevier Ltd. All rights reserved.