Assessing conceptual understanding of foundational topics before instruction on higher-order concepts can provide chemical educators with information to aid instructional design. This study provides an instrument that can be used to identify students' alternative conceptions regarding thermochemistry concepts. The Thermochemistry Concept Inventory (TCI) has been developed for use in formative and summative assessments. Items on the TCI were developed and refined through qualitative evaluation as well as b and pilot tests. Data for the psychometric analysis of the TCI was gathered during a large data collection (N = 1330) and was analyzed using the Rasch model. Supporting evidence for response process validity, structural validity, and reliability were collected. Testing data shows that the TCI is unidimensional and has acceptable fit to the Rasch model. In support of prior qualitative studies, option probability curves support the response process validity and reliability of the items. With exception of one item, when used in summative testing, all items displayed good item functioning. Test-level analysis indicated that the TCI was well targeted to the ability of students in our testing samples. Performance data from different groups shows that the TCI is a measure of overall student ability, providing evidence of concurrent validity.