In this study the magnesium diffusion behaviour was studied in pellets with fine and course olivine, with and without additional fine quartzite (<20 mu m) after isothermal reduction at 1 000-1 300 degrees C. It was found that, by using a fine olivine (<38 mu m) the whole magnesium content of the olivine was dissolved evenly in the wustite and in the slag, already at 1 000 degrees C, in agreement with the equilibrium tie-lines of the FeO-MgO-SiO2 phase diagram. This lead the liquid slag to precipitate into fayalitic olivine and the Al, Na, K, Ca, P-content to enrich in remaining inclusions in the olivine. This crystallization did not occur in the sample with only bentonite addition, or in the sample with unreactive olivine at these temperatures. However, with further addition of fine quartzite, the slag of the sample with coarse olivine also crystallized. In the samples reduced at 1 000-1 100 degrees C, magnesium gradients could be detected in the slag phase around coarse olivine particles until entering the interaction volume of an interfering particle at around similar to 600 mu m, or occasionally at distances of more than 1 mm. For the coarse olivine the main rise in magnesium occurs at 1 200 degrees C when the olivine particle cores begin to dissolve. The dissolution of all magnesium of the 2.5% olivine addition during oxidation lead to 6.5% Mg in the crystallized slag phase. The increase in melting point resulting from this compared to fayalite with no magnesium is similar to 50 degrees C, according to thermodynamic calculations. .