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The conditions for void initiation in aluminum based metal matrixcomposites in under- and peak-aged conditions with a varying volume fraction of aluminaparticles are studied. A digital image analysis system is applied to perform a quantitativeanalysis of the fracture surfaces from stereo image pairs. The crack tip openingdisplacement in the moment of void initiation, CODvi, is determined as a function of thedistance and the angle of the alumina particles nearest to the crack tip. For each individualparticle, the stress tensor at the moment of void initiation is calculated from the solution ofthe HRR-field. A mean-field theory is employed to calculate the maximum normal stresses,σ max part.vi, in the particle. An extremely high scatter of the CODvi-values is found; theσ max part.vi-values show a much lower scatter. The reasons for that are discussed.