Assessing student learning outcomes has been an integral and central component of schools and academic institutions, both as a measure of how well the school is performing and how well individual students are progressing. However, standardised tests might not be able to fully capture holistic aspects of learning such as self-regulatory competencies, motivation and interest. The Structure of Observed Learning Outcomes (SOLO) taxonomy has been used to examine the quality of learning outcomes. Based on findings from the current study in a STEM-based learning curriculum, a new model, the Structure of Observed Learning and Interest Outcomes (SOLIO) taxonomy, is proposed to capture student interest development as well as learning outcomes for a more holistic assessment of learning.