In this paper the dynamics of the magnetization reversal process in perpendicularly biased [20 Å Pt/5 Å Co]3 Å Pt/100 Å IrMn/20 Å Pt multilayers with different Pt insertion layer thickness (0 Å ≤ t ≤ 12 Å) is studied. The insertion of 1 Å thick Pt enhances the exchange bias field (Hex) and for t > 3 Å Hex decreases exponentially with increasing Pt layer thickness. We show by magnetization relaxation measurements and direct observation of magnetic domains that magnetization reversal takes place by the nucleation of isolated cylindrical domains with a different nucleation site density in the forward and backward branches of the hysteresis loop. All the results were quantitatively analyzed using the Fatuzzo model for the dynamics of domain reversal processes. The activation energies for magnetization reversal by domain nucleation and domain propagation were determined.