The presence of Mo5Si3 in MoSi2 preforms hinders the reactive infiltration of aluminum. To understand the role of Mo5S3, the kinetics of aluminum infiltration into pure Mo5Si3 is studied. Irrespective of the initial composition (MoSi2 or Mo5Si3) of the preform, the final product always contains Mo(Al,Si)(2). However, the aluminum content in the two cases is different: when the preform is MoSi2, the aluminum content is 14-18 at.%, and, when the preform is Mo5Si3, the aluminum content is 25-27 at.%. The activation energy for the reactive infiltration of aluminum into the Mo5Si3 preform is similar to26 kJ/mol.