This study involves the development of a three-tier diagnostic test to measure high school students' understanding of states of matter concepts. The States of Matter Diagnostic Test (SMDT) is a 19-item three-tier diagnostic test consisting of three-tier items for assessing students' understanding of states of matter concepts. The SMDT was administered to 195 10th grade high school students in the pilot study and 102 10th grade high school students in the main study. Cronbach alpha reliability indexes for the SMDT were estimated to be .78 and .83 for the pilot and main study, respectively. Point-biserial coefficients ranged from .20 to .69 with an average of .44 for the pilot study and with an average of .49 for the main study.