The DynaLearn interactive learning environment enables learning by having learners create conceptual models of system's behavior. This paper reports on exploratory evaluation studies using the DynaLearn software, carried out with learners studying environmental science. Two three-day modeling sessions were conducted in two consecutive years with two students exploring the evolving prototype of the software. The learners worked on assignments designed to achieve specific learning goals. To investigate conceptual changes on behalf of the learner, a set of parameters was applied for semantic text analysis of written pre- and posttests. The evaluation results show key changes occurring in knowledge structure and content in both years for both students. Indications of an effect of prior knowledge on the magnitude of conceptual change were found. The results confirm the potential of DynaLearn for inducing causal and interlinked understanding of environmental systems.