Precise determinations of sulfate and chlorinity were made on SIPRE core samples of natural sea ice. One ice was a multiyear ice that had remained attached to T-3 (Fletcher's Ice Island) for over 20 years and the other was a shore-fast annual ice collected near Barrow, Alaska. Our results show sulfate:chlorinity ratios different from the seawater ratio. A mixing model shows that on warming and aging, sulfate and chlorinity are lost in a constant ratio, indicating conservative behavior. Fractionation in sea ice appears to be determined in the freezing process and further enrichment does not take place with aging. The effects of sulfate fractionation in sea ice are too small and the residence times of both sea ice and arctic Ocean surface water are too short to permit detectable modification of seawater composition by freezing.