A sample of 100 students from junior high school physical science, high school chemistry, and introductory college chemistry were examined for understanding of five chemistry concepts. The concepts addressed were chemical change, dissolution of a solid in water, conservation of atoms, periodicity, and phase change. The amount of experience with the concepts (grade level) and reasoning ability (developmental level) were examined as possible sources of variation in student understanding. Differences in understanding with respect to grade level were found to be significant for the concepts of chemical change, dissolution of a solid, conservation of atoms, and periodicity. However, few of the students in the college chemistry sample exhibited sound understanding of chemical change, periodicity, or phase change. The use of particulate terms (atoms, ions, molecules) increased across the grade levels. Reasoning ability proved to be a significant factor for student understanding of conservation of atoms and periodicity. An examination of the number and types of misconceptions across the grade levels revealed several interesting patterns and suggested sources for the students' alternative conceptions.