The relationship between the isotopic composition of seawater and salinity and the variation of the isotopic composition of precipitation with latitude have frequently been used to identify freshwater sources and to tag water masses. Underlying these applications is the assumption that the isotope-salinity relationship is affected only by the mixing of salt and fresh waters. Simple box models are used to show how Δ18O-salinity (Δ-S) relationships in the upper water column vary seasonally in areas where sea ice forms or melts. During melting, the Δ-S relationship takes a number of different shapes, but it returns to a straight line late in the melting season. During freezing, the Δ-S shape stays linear, but its slope varies. The models show that ice-related processes can produce very large changes in the Δ18O value of apparent freshwater components (Δ18OS=0), the parameter usually used to label water masses. At the end of the melting season the Δ18OS=0 value may be 10¿/oo more positive than at the beginning; at the end of the freezing season, Δ18OS=0 may be more than 20¿/oo more negative than at the beginning. Thus the models provide explanations for both anomalously heavy Δ18OS=0 values in the Labrador Current (-7.6¿/oo) and anomalously light values in the Canadian arctic archipelago and the East Greenland Current (≈-50¿/oo). They can also explain the negative slope occasionally observed in Δ-S plots. Caution must be exercised in using Δ18OS=0 to identify source waters derived from the Labrador Sea, which receives significant amounts of freshwater from Baffin Bay and Hudson Strait which, in many cases, have been altered by more than a single season of melting/freezing activity.