Using the US national sample from the 1995 Third International Mathematics and Science Study (TIMSS), this study examined students' competence levels in understanding the matter concept at grades 3, 4, 7, 8 and high school graduation, and compared them to the expectations in the US national science education standards. It was found that third-grade students were developing understanding on mixtures, and fourth-grade students were developing understanding on separating mixtures; seventh- and eighth-grade students were only at the beginning level of differentiating chemical properties from physical properties; they were not ready for the particulate model of chemical change. High school physical science specialization students were still at the developing level of understanding kinetic and atomic models of chemical and physical changes; they may not be able to master those theories. The findings suggest that the "Benchmarks for Science Literacy" and "Atlas for Science Literacy" may have overestimated the competences of elementary, middle school, and high school students.