Abstract. In recent times, there has been an increased utilization of serverside XSLT systems as part of e-commerce and e-publishing applications. For the high volumes of data in these applications, effective optimization techniques for XSLT are particularly important. In this paper, we propose two new optimiza-tion approaches, Specialization Combination and Specialization Set Compac-tion, to help improve performance. We describe rules for combining specialized XSLT stylesheets and provide methods for generating a more compact speciali-zation set. An experimental evaluation of our methods is undertaken, where we show our methods to be particularly effective for cases with very large XML in-put and different varieties of user queries.