The results of measuring relative brine content (Q
lm) in the liquid phase of frozen sea water at temperatures between −2°C and −43°C with variable rate and direction of temperature change are presented. Various pulsed and steady-state nuclear magnetic resonance (NMR) methods are used and the results are compared with data in the literature. Temperature dependences of Q
lm in sea water with dry NaCl added are given. Quasi-hysteresis corresponding to the crystallization range of certain seawater salts, mainly NaCl, which partially precipitates as the crystalline hydrate NaCl·2H2O at temperatures below −23°C, is detected in the temperature dependence of Q
lm. The reasons why Q
lm values, under various experimental conditions, differ from data in the literature are explained. Brine salinity (S) is calculated from the relative intensity of NMR signals of Q
l, and agrees well with previously published data over a wide temperature range. It is shown that determining Q
lm from base measurements of Q
l in ice does not require information about brine salinity. Empirical equations for calculating brine content at different sea-water temperatures and salinity are given.