We examined whether limber pine and skunkbush sumac individuals co-existing on the Pawnee Buttes of northeastern Colorado have the ability to access and utilize deep soil water resources. The δ18O signature of source water to the plants (deep soil water and precipitation) and plant cell water were measured in June 2000. The δ18O signatures of the two woody plant species were not significantly different from each other. However, the average δ18O signature of skunkbush sumac (−9.4‰) differed from the δ18O signature of deep soil water (−13.0‰), while the average δ18O signature of limber pine (−11.3‰) did not. Variability in the δ18O signature across plots and between individuals within a plot was relatively high for both species. These results suggest limber pine and some individuals of skunkbush sumac are able to access and utilize deep soil water resources at this site. This ability may confer an ecological advantage to these plants given the semi-arid climate of the site.