As the United States has begun to lag behind other developed countries in performance on mathematics and science, researchers have sought to explain this with theories of teaching, knowledge, and motivation. We expand this examination by further analyzing a measure of interest that has been linked to student performance in mathematics and intention to enroll in advanced mathematics classes. Using Item Response Theory (IRT), the purpose of this study was to evaluate the psychometric properties of the Mathematics Interest Inventory (MII). Six hundred sixty-six students (47.6% Hispanic, 48.2% Male) formed the sample. Results revealed minimal differential item functioning (DIF) between genders. Substantial DIF emerged between Hispanic and White students. Implications of the differential functioning and the measurement of interest in mathematics are discussed.