The aim of this study is to investigate Emiratii high school students' understandings of stoichiometry, their use of metacognitive strategies, and the influence of students' use of metacognitive strategies on their understandings of stoichiometry. Two instruments were used in this study, the first to measure students' understandings of stoichiometry and the second to measure students' use of metacognitive strategies. One hundred and sixty-two 11th grade students participated in this study, 80 boys and 82 girls. The results showed that students' understanding of stoichiometry was low and students used five metacognitive strategies: awareness of cognition, planning, monitoring and self-checking, self-appraisal and engagement in task. Planning was the most used strategy and could predict students' success in displaying sound understanding of stoichiometry. The results also showed that students' overall use of metacognitive strategies predicts their understanding of stoichiometry. The study argues that students should be taught how to use metacognitive strategies to improve their understanding of stoichiometry in particular and chemistry in general. (Contains 7 tables and 1 figure.)