Research on computer-based adaptive learning environments has shown exemplary growth. Although the mechanisms of effective adaptive instruction are unraveled systematically, little is known about the relative effect of learners' perceptions of adaptivity in adaptive learning environments. As previous research has demonstrated that the learners' view towards a learning environment strongly influences their learning outcomes and learning process, it can be discussed whether program-defined adaptivity is not only effective because of the underlying learner models, but also because the adaptivity is perceived and experienced as such by the learners. In this study, we apply the cognitive mediational paradigm and hypothesize that perceptions of adaptivity mediate the relation between adaptive instruction and learners' motivations and learning outcomes. The results do not fully support the claim of the cognitive mediational paradigm. Both adaptivity and perceptions were related to motivation, but learners' perceptions did not act as a mediating variable.