Nucleation activity of unsized and differently sized glass fibers during the crystallization of polypropylene from melt was investigated by polarizing light microscopy and DSC. Depending on the type of surface treatment, glass fibers were shown to exhibit different nucleating effects, evaluated by induction rime of crystallization. crystallization onset temperature as well as half-time of crystallization in model composites with 50% wt glass fibers. Predominant nucleation activity was found for glass fibers sized with polypropylene compatible dispersion containing polyurethanes. However, according to the results of DSC measurements, unsized glass fibers slightly depressed the nucleation of polypropylene. Using the approach of Dobreva et al., the activity of the fibers towards heterogeneous nucleation during nonisothermal crystallization was evaluated. (C) 2001 Elsevier Science Ltd. All rights reserved.