The aim of this study was to examine how students used evidence in argumentation while they engaged in argumentive and reflective activities in the context of a designed learning environment. A Web-based learning environment, SOCRATES, was developed, which included a rich data base on the topic of climate change. Sixteen 11th graders, working with a partner, engaged in electronic argumentive dialogs with classmates who held an opposing view on the topic and in some evidence-focused reflective activities, based on transcriptions of their dialogs. Another sixteen 11th graders, who studied the data base in the learning environment for the same amount of time as experimental-condition students but did not engage in an argumentive discourse activity, served as a comparison condition. Students who engaged in an evidence-focused dialogic intervention increased the use of evidence in their dialogs, used more evidence that functioned to weaken opponents' claims and used more accurate evidence. Significant gains in evidence use and in metalevel communication about evidence were observed after students engaged in reflective activities. We frame our discussion of these findings in terms of their implications for promoting use of evidence in argumentation and in relation to the development of epistemological understanding in science.