In this article, we present a theoretical framework for mixed reality (MR/XR) self-determined learning to enhance ecological literacy in free-choice educational settings. The framework emerged from a research study in New Zealand which aimed to explore how learning experiences which incorporate mobile technologies within free-choice learning settings can be designed to enhance learner development of marine ecological literacy. An understanding of how mobile technology can be integrated into the teaching and learning of sustainability education that incorporates free-choice learning contexts, such as visitor centres, is of strategic importance to both education outside the classroom and adult learning. Following a design-based research methodology, the framework is presented in the form of a set of design principles and guidelines, informed by key theories in ecological literacy and free-choice learning, heutagogy, bring your own device and self-determined learning. We briefly describe how the framework provided the foundation for an educational intervention. This paper aims to assist researchers and developers of MR/XR immersive learning environments to consider design principles and processes that can enhance learning outcomes within free-choice settings, such as museums and visitor centres.