This study examined how students in a Hong Kong high school used Diigo, an online annotation tool, to support their argumentative reading activities. Two year 10 classes, a high-performance class (HPC) and an ordinary-performance class (OPC), highlighted passages of text and wrote and attached sticky notes to them to clarify argumentation structures and to represent and share argumentation processes. Analysis of annotations revealed that highlighting was the most frequently used online annotation feature. The HPC made significantly more sticky notes and used them more to annotate terms, claims and judgments than the OPC. The study suggests that Diigo is a promising tool for supporting reading-to-argue. The findings may be of value to teachers and researchers in designing online annotation tools that more effectively foster the process of reading-to-argue. (Contains 4 tables and 1 figure.)