The 18O – mean annual temperature relationship, which is known in organic matter, presents some anomalies in Sphagnum. A study of water flow and isotopic characteristics of an arctic mire (Leaf River, Ungava Peninsula) has been undertaken to explain these anomalies. The isotopic compositions of the water (18O, 13CO2, 3H) permit the determination of some hydraulic characteristics of a peat on permafrost: (1) evaluation of the lag time between a rainfall and the discharge; (2) evaluation of the residence time of the water; and (3) subsequent mixing parts. A relationship between the isotopic characteristics and the ombrotrophic or minerotrophic nature of the mire is observed. A δ18O value of −11‰ appears in the ombrotrophic zone and a value of −17‰ in the minerotrophic zone. The relative enrichment in 18O of the first one indicates a shorter residence time. The isotopic changes subsequent to rainfalls (with δ18O = −12.2, −14.5, and −19.1‰ differ in time and space: (1) a 2‰ change is observed within 24 h in the minerotrophic zone; (2) a 6‰ change appears almost instantaneously in the ombrotrophic zone. 3H values indicate that neither the meltwater from winter snow (with low 3H content), nor the meltwater from permafrost participates in the subsurface flow.