This study examines Indonesian students' understanding of conservation of matter, balancing of equations and stoichiometry. Eight hundred and sixty-seven Grade 12 students from 22 schools across four different cities in two developed provinces in Indonesia participated in the study. Nineteen teachers also participated in order to validate the 25-question survey used with all students. Significant differences in student success in answering specific questions occurred when comparing high-achievement and low-achievement schools. However, in general, student understanding of this fundamental principle in chemistry was low. The study found that the average score for all students on the survey was 41%. The findings suggest that students are most successful in solving problems used by teachers and textbooks that are algorithmic-based (i.e., stoichiometry). As there were no strong positive correlations between student performance on conceptual questions and algorithmic questions, we suggest that further research should focus on teaching practices and curricula that support the development of the students' conceptual understanding. (Contains 3 tables and 4 figures.)