Though the Chemistry Self-Concept Inventory (CSCI) was developed to study one aspect of the affective domain in college chemistry students, the instrument on which it was based, the Self-Description Questionnaire III, was developed for use with late adolescents. As such, we explored data generated from administering the CSCI to high school students in seven teachers' classrooms in four schools. This paper presents findings which describe the validity and the reliability of the data obtained from the CSCI when used with high school students. The validity of the data is supported by the results of an exploratory factor analysis, relationships between subscales and chemistry knowledge, and comparisons among different groups of students. Evidence supporting the reliability of the data is provided by internal consistency values for each of the subscales. The results presented in this paper demonstrate the usefulness of the CSCI with a high school population. This instrument may be used by teachers to better understand students' beliefs about themselves with respect to chemistry learning. Implications for research include the utility of the CSCI with high school chemistry students.