Is adaptive scaffolding effective in facilitating students' ability to regulate their learning of complex science topics with hypermedia? We examined the role of different scaffolding instructional interventions in facilitating students' shift to more sophisticated mental models as indicated by both performance and process data. Undergraduate students (N=51) were randomly assigned to one of three scaffolding conditions (adaptive scaffolding [AS], fixed scaffolding [FS], and no scaffolding [NS]) and were trained to use a hypermedia environment to learn about the circulatory system. Pretest, posttest, and verbal protocol data were collected. Findings revealed that the AS condition facilitated the shift in learners' mental models significantly more than did the other comparison conditions. Participants in the AS condition regulated their learning by activating prior knowledge, monitoring their emerging understanding by using several strategies, and engaging in adaptive help-seeking. Learners in the FS and NS conditions were less effective at regulating their learning and exhibited great variability in self-regulation of their learning during the knowledge construction activity. We discuss how the findings can be used to inform the design of MetaCognitive tools--adaptive hypermedia environments designed to foster students' self-regulated learning of complex topics.