Abstractδ18O and δD values (% SMOW) were measured on 159 samples from the active layer and permafrost on peat plateaus and from the adjacent fen at sites about 50 km north of Tuchitua along the Robert Campbell Highway in southeastern Yukon Territory. The regression lines for the samples from the active layer and the permafrost were very similar, which suggests that they both contained mainly water from meteoric sources. The water samples from the fen produced results which gave a noticeably different regression line, so that it appears that the icy core of the peat plateaus is the result of downward penetration of precipitation into permafrost rather than freezing water migrating from the surrounding fen. These relationships appear to be constant throughout the study area, and there is no evidence of the ice being from an earlier, colder period. The precipitation in this area of the Yukon Territory shows a δD versus δ18O behaviour somewhat like that reported for Whitehorse in the literat