An aluminum/MgAl2O4 in situ metal matrix composite has been synthesized using silica gel containing similar to 98% SiO2 in an Al-5Mg alloy. The thermodynamics and kinetics of MgAl2O4 formation have been discussed in detail. A transition phase of composition between MgO and MgAl2O4 has been detected in the SEM-EDS analysis of the particles extracted from the composite by a 25% NaOH solution. This confirms the gradual transformation of MgO to MgAl2O4 by the reaction 3SiO(2)(s)+2MgO(s)+4Al(l)-> 2MgAl(2)O(4)(s)+3Si(l). The stoichiometry, n, of MgAl2O4 has been found to sustain close to 1 and the crystallite growth of MgAl2O4 has been stopped at D similar to 30 nm in the composites held at 750 degrees C up to 10 h.