Inline measurement of the supersaturation is important to support process control and for the evaluation of crystallization experiments. Especially, when different hydrates or polymorphs of the crystallized component occur, very precise measurement is required. For example MgSO4 center dot 7H(2)O-MgSO4 center dot 12H(2)O lines are quite close, and when operating under eutectic freeze crystallization (EFC) or cooling crystallization both salts might be produced. Therefore conductivity and refractive index inline measurement methods are investigated and compared for MgSO4 solution having concentration range between 16 and 22wt% and temperature range between 10 and -5 degrees C. Conductivity measurements are correlated to concentrations and temperature with the empirical Casteel-Amis equation and refractivity index measurements are correlated to concentrations and temperature with a developed empirical model. The metastable lines for ice and salt of MgSO4 system is drawn based on the measurements at the onset of crystallization upon cooling of several solution concentrations. In the working range of EFC, the relative supersaturations are calculated to be sigma(icemax) = 0.2 and sigma(saltmax) = 0.23: The accuracy for conductivity measurement in relation to the metastable zone width is roughly calculated to be around 20% and for refractive index measurements, 3%. (C) 2004 Elsevier B.V. All rights reserved.