The formation and growth of MgAl2O4 spinel crystals on a single crystal MgO substrate submerged in a 40% CaO, 40% SiO2 and 20% Al2O3 slag was directly observed using high temperature microscopy. This showed that the crystals initially form on the MgO surface, but may break off and be carried out into the liquid slag. Still pictures extracted from digitally recorded images were used to measure the size of these crystals at 1420, 1440 and 1460oC as a function of time. Growth of the crystals was found to follow the parabolic rate law, with rates increasing with temperature. An estimate of the activation energy made from the data (564 kJ mol-1) was found to be comparable with previously published results from different types of experiments on spinel formation.