Sorbitol-type nucleating agents used as clarifiers for polypropylene (PP) show a complex interplay of phase and crystallization behavior. Crystallization measurements were made by fast scanning calorimetry (FSC) using cooling rates between 10 K s-1 and 4000 K s-1 in order to study the influence of cooling rates realistic to those used in processing. These measurements were combined with differential scanning calorimetry (DSC) measurements made at slow cooling rates (10 K min-1). The optical properties are characterized by haze measurements. For this investigation the commercially available clarifiers 1,3:2,4-bis(3,4-dimethylbenzylidene) sorbitol (DMDBS) and 1,2,3-tridesoxy-4,6:5,7-bis-O-[(4-propylphenyl) methylene]-nonitol (TBPMN) were added to PP in various amount up to 0.6 wt.%. At relatively slow cooling rates only a single crystallization process was observed. In contrast, fast cooling causes a complex crystallization behavior with up to four different crystallization processes. It was found that the temperature of the dominant crystallization process during fast cooling correlates with the optical properties from haze measurements. Finally, the cooling rate dependence of the different crystallization processes measured by FSC is discussed in terms of nucleating efficiency of the clarifiers. In contrast, results of conventional DSC measurements cannot be extrapolated in a simple way for describing the nucleation activity of nucleating agents at cooling rates which are typical of those used in processing.