This qualitative case study explored how exposure to Process-Oriented Guided Inquiry Learning (POGIL) improves 11th grade physical sciences learners' competency in solving stoichiometry problems. Two township schools of low socioeconomic status in Pretoria, South Africa, were conveniently and purposively sampled. Two classes (N = 48) composed of multicultural black learners of mixed gender participated in the study. Data were collected at each school using a pre-test, post-test and video-recorded classroom observations. The Interactive-Constructive-Active-Passive theory served as a conceptual and analytical framework to understand the nature of student engagement during the POGIL activities. High student engagement could contribute to improved problem-solving competency in the post-test. The findings from the classroom observations and recorded group discussions served as evidence of the learners' engagement. The learners collaboratively engaged in the analysis, synthesis and evaluation of ideas, reasoning and justifying their suggestions in solving POGIL tasks. Interactions were largely characterised as interactive and constructive modes of engagement, both facilitating deep processing strategies. Regarding data from the tests, problem-solving competency was classified at four levels based on Bloom's taxonomy. The pre-test results indicated a lack of competency in solving complex stoichiometry problems. ANOVA indicated that the differences in the frequencies of the competency levels found in the pre- and post-test are statistically significant. High student engagement could have contributed to improved problem-solving competency in the post-test, although more improvement could be made in terms of problem solving competency in questions introducing more complexity and higher cognitive demand such as multistep calculations. The study shows how using POGIL in high school science may empower learners with enhanced problem solving skills.