The response of the MgO-20 and 30 mol% Al2O3 Compositions to rapid solidification has been studied. The oxides were twin-roller quenched and the resulting flakes were characterized by X-ray diffraction and transmission electron microscopy. The results indicate that metastable extensions of spinel and periclase occurred and the microstructural pathway was determined from the final microstructure. The flakes having MgO-20 mol % Al2O3 show a dendritic structure consisting of periclase and spinel. In the MgO-30 mol % Al2O3 composition, the liquid transforms to spinel partitionlessly. The spinel is believed to undergo decomposition by a modulation in composition, and the resulting microstructure consists of spinel and periclase. Kinetic and thermodynamic aspects of phase selection have been rationalized based on the metastable extensions of the different phase fields in the phase diagram. It has been proposed that composition fluctuations in spinet are stabilized because of the formation of disordered phases with a continuous range of order parameter on the tetrahedral sublattice.