The application of mechanoluminescence (ML) for online monitoring of grinding was tested. In stirred media mills ML is induced by the stressing of particles between mill beads. The light impulses accompanying the stress events are analyzed. The primary particles and aggregates exhibit a different behavior during impact loads. Mechanical stress applied to solid particles leads to the fracture of particles. Light emission is intensive in this case. Application of stress to aggregates mostly leads to the cracking of the solid bridge bonds between the primary particles. As a consequence, ML is not so intensive. Based on this fact, the breakage of aggregates was online monitored from the time dependence of the stress frequency. The reduction of light impulse frequency caused by the change in particle structure with grinding time was observed.