Writing-to-learn assignments typically foster deep comprehension of learning contents. Journal writing, in particular, promotes the application of learning strategies, which promote learners’ comprehension, interest in a topic and ability to critically reflect on learning contents. Against this background, we conducted two longitudinal field studies. In Study 1, twenty-one students wrote learning journal entries after their biology lessons. After the intervention period, they showed better scores in comprehension, interest and critical reflection than the control class, in which students (n=25) completed other homework assignments. Mediation analyses revealed a domino effect: Journal writing improved comprehension, which led to raised interest, which resulted in superior critical reflection. Study 2 further investigated the role of learners’ interest in improving critical reflection. Students in the experimental condition (n=13) received a personal-utility prompt in addition to cognitive and metacognitive prompts to support journal writing. In the control group (n=11), students only received cognitive and metacognitive prompts. The experimental group showed higher interest scores after the intervention period and a better quality of critical reflections on a bio-ethical issue than the control group. Together, these studies illustrate the potentials of journal writing for improving learners’ comprehension, their interest and ability to critically reflect on complex scientific issues.