Solute and runo time-series at Finsterwalderbreen, Svalbard, provide evidence for considerable basal routing of water and the existence of at least two contrasting subglacial chemical weathering environments. The hydrochemistry of a subglacial upwelling provides evidence for a snowmelt-fed subglacial reservoir that dominates bulk runo during recession ¯ow. High concentrations of Cl 7 and crustal ions, high pCO 2 and ratios of [ * SO 2À 4 /( * SO 2À 4 HCO À 3 )] close to 0 . 5 indicate the passage of snowmelt through a subglacial weathering environment characterized by high rock:water ratios, prolonged residence times and restricted access to the atmosphere. At higher discharges, bulk runo becomes dominated by icemelt from the lower part of the glacier that is conveyed through a chemical weathering environment characterized by low rock : water ratios, short residence times and free contact with atmospheric gases. These observations suggest that icemelt is routed via a hydrological system composed of basal/ice-marginal, englacial and supraglacial components and is directed to the glacier margins by the ice surface slope. Upwelling water ¯ows relatively independently of icemelt to the terminus via a subglacial drainage system, possibly constituting ¯ow through a sediment layer. Cold basal ice at the terminus forces it to take a subterranean routing in its latter stages. The existence of spatially discrete ¯ow paths conveying icemelt and subglacial snowmelt to the terminus may be the norm for polythermal-based glaciers on Svalbard. Proglacial mixing of these components to form the bulk meltwaters gives rise to hydrochemical trends that resemble those of warm-based glaciers. These hydrochemical characteristics of bulk runo have not been documented on any other glacier on Svalbard to date and have signi®cance for understanding interactions between thermal regime and glacier hydrology.