Early research on using the World Wide Web indicated that middle school students did not explore much and used Web tools naively. In response to these challenges, an on-line research engine, Artemis, was designed to provide a permanent workspace and allow students access to preselective on-line resources. This study investigated the depth and accuracy of sixth-grade students' content understandings as well as their use of search and assess strategies when they used on-line resources via Artemis. Eight student pairs from two science classes experienced support from teachers and used scaffolded curriculum materials while completing four on-line inquiry units during 9 months. Multiple sources of data were collected, including video recordings of students' computer activities and conversations, students' artifacts and online postings, classroom and lab video recordings, and interview transcripts. Analyses of data showed that students constructed meaningful understandings through on-line inquiry, although the accuracy and depth of their understandings varied. The findings suggest that students might develop accurate and in-depth understandings if they use search and assess strategies appropriately, if resources are thoughtfully chosen, and if support from the learning environment is extensively provided. This research lends evidence to questions regarding the value of students engaging in on-line inquiry. ß