We assessed the applicability of end-member mixing analysis (EMMA) in a high mountain catchment (Central Pyrenees) to quantify the contributions of snowpack, rainwater, lake water, and shallow and deep groundwater to surface flow. The sampling was conducted from March to November 2010, using an automated system that allowed stream water sampling with high temporal resolution (from 10 minutes in the rising limb to eight hours in the receding limb) to capture all discharge events in detail. We measured conductivity, alkalinity, pH, dissolved inorganic carbon and dissolved organic carbon (DOC), Na+ , K+ , Ca2+, Mg2+, Cl− , SO4 2−, NO3 − , NH4 + , NO2 − , and dissolved reactive silica (DRSi). Using a hydrological diagnostic tool set prior to the EMMA, we determined that only Na+ , Ca+ , DRSi, and DOC could be used as tracers. Three end members were defined (atmospheric water, shallow groundwater, and deep groundwater), and the EMMA results showed that groundwater (deep plus shallow) was an important (~50%) year-round contributor to the surface flow, with peaks reaching > 70% during rainfall episodes.