The sea ice cover of high latitude oceans contains concentrated brines which are the site of in-situ chemical and biological reactions. The brines become supersaturated with respect to mirabilite (Na2SO4·10H2O) below −6.4 °C, and the associated removal of Na+ and View the MathML source from the brine results in considerable non-conservative changes to its composition. The changes are reflected in the brine salinity, which is a fundamental physico-chemical parameter in the sea ice brine system. Here, measurements of electrical conductivity and brine composition in synthetic sea ice brines between −1.8 and −20.6 °C, obtained during a comprehensive investigation of the brine-mirabilite equilibrium at below-zero temperatures reported elsewhere, are combined with modelled estimates to assess the behaviour of the absolute (SA) and practical (SP) salinities of sea ice brines. Results display substantial divergence of SP from SA below −6.4 °C, reaching a 7.2% difference at −22.8 °C. This is shown to create inaccuracies when SP is assumed to be equivalent to SA, firstly by misrepresenting the conditions inhabited by sea ice biota, whilst also creating errors in the calculation of physical sea ice parameters. Our measured and modelled data are used to refine the SA − T relationship for sea ice brines, implicit of mirabilite precipitation, which is crucial in estimating brine properties in absence of salinity data. Furthermore, because SP is the parameter measured in field studies, we provide an SP − T relationship for sea ice brines to −22.8 °C, which aids in explaining the trends observed in available SP − T data from sea ice brines in the Southern Ocean, demonstrating the importance of the mirabilite-brine equilibrium in natural sea ice. Finally, we initiate the development of a conversion factor for the estimation of SA from SP measurement in sea ice brines, and produce an equation that can calculate SA from modelled brine density. This work ultimately highlights careful consideration of salinity concepts when applied to the sea ice system.