Thermally expandable core/shell particles with a poly(acrylonitrile-co-methacrylonitrile) shell and a hydrocarbon core (blowing agent) have been prepared by suspension polymerization. The objective of this study was to gain a deeper understanding of the parameters determining the expansion properties of these microspheres. it was found that the amount, the boiling point, and the structure of the blowing agent are important parameters for the expansion properties. For example, a higher maximum expansion was reached when using bulkier blowing agents and thus a lower diffusion rate through the polymer shell. Further, the amount and structure of the crosslinker were also found to be essential for the expansion properties. For this particular system, it was found that a dimethacrylate-functional crosslinker gave significantly better expansion when compared with diacrylate- or divinylether-based crosslinkers. Beside these parameters, it was also observed that the particle-size distribution influence the expansion properties of the microspheres. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 384-392, 2010