The aim of this study has been to improve the technological process of obtaining magnesium oxide from seawater by sub-stoichiometric (80 %) precipitation with dolomite lime. Taking into account that the flaw of sub-stoichiometric precipitation is the increased B2O3 content in the final product, the experimental part examines in detail the method of treating the magnesium hydroxide precipitate for different operating conditions in order to determine the optimum rinsing method. Examinations relate to the selection of the rinsing agent of a specific pH value, the sequence of application of the rinsing agent, and determination of the minimum number of rinses both in decantation and in rinsing on filter paper with the aim of obtaining magnesium oxide with a low B2O3 content. Based on the results obtained, the optimum method of rinsing the magnesium hydroxide precipitate has been found, which reduces both the B2O3 content and the CaO content in the calcined magnesium oxide. The optimum method of rinsing of the Mg(OH)2 precipitate is the combined rinsing method (2+3) (rinsing 2 times with distilled water of pH of 5.86 and 3 times with alkalized distilled water of pH of 12.50) in the process of decantation, and 5 times with fresh distilled water of pH of 5.86 on filter paper with regards to the CaO and B2O3 content and the quantity of alkalized water spent. The calcined magnesium oxide (80% precipitation) obtained by calcination of the magnesium hydroxide precipitate rinsed according to the method described contains 98.68 mass % MgO, 0.83 mass % CaO and 0.1888 mass % B2O3 and can be used as the initial raw material in obtaining the sintered magnesium oxide. The quantity of sintered magnesium oxide is affected by the composition of initial raw materials, and consequently the chemical composition of seawater (from the location of the Oceanographic institute in Split) was examined as well as the chemical, mineralogical and granulometric properties of dolomite lime obtained by calcination of dolomite (from the ðipalo – Sinj location). Then the process of sintering of magnesium oxide was examined and the effect of the TiO2 addition ( 1, 2 5%) on its microstructure at sintering temperatures of 1400, 1500 and 1700 oC, with the duration from 1 to 4 hours. The aim of the study has also been to determine how to make the seawater-derived magnesium oxide obtained by sub-stoichiometric 80% precipitation of magnesium hydroxide the efficient refractory material, i.e. how to obtain densified polycrystalline sample of appropriate microstructure with as low sintering temperature and small percent of added TiO2 as possible. Already at low sintering temperatures (1400 oC) with the addition of 2 mass % TiO2 or5 mass % TiO2, depending on the duration, the requirement has been met for obtaining quality sintered MgO (containing ≤ 0.05 mass % B2O3) that is used in the refractory industry. The density of samples sintered at lower temperatures (< 1500 oC) amount up to 96.50% and at higher temperatures they amount up to 99.50% of the theoretical density ( 3.576 g cm). The microstructure of the MgO samples with the addition of TiO2 becomes more compact already at lower temperatures and shorter sintering duration due to the increased periclase grain (MgO). CaTiO3 and MgTiO4 separate in the form of a thin layer over the periclase grain boundary surface during cooling. At higher temperatures the appearance of a small quantity of the liquid phase additionally intensifies sintering and growth of the periclase grain. The laws governing the thermodynamics of irreversible processes have been applied to reactions of formation of Ca2B2O5, CaTiO3 and MgTiO4 in order to determine the phenomenological equations of activated sintering of seawater-derived magnesium oxide. The relations obtained make it possible to use computing to predict the B2O3 content in sintered MgO samples as well as the content of CaTiO3 and Mg2TiO4 formed relative to temperature and duration of isothermal sintering and the addition of TiO2. The mathematical models established make it possible to control the activated sintering of seawater-derived magnesium oxide in the temperature range 1400 − 1700 oC, for the duration of 1 − 4 hours.