The Inventory of Emotions Experienced by Adolescents when Solving Mathematical Tasks (INETAM, for its acronym in Spanish), measures four influential emotions related to Math performance: Enthusiasm, Frustration, Enjoyment, and Boredom. Content validity, construct validation and item calibration analyses were performed to obtain evidence of its validity, using a sample of 448 adolescents enrolled in ninth grade. Factor analysis showed adequate reliability coefficients and fit indices. Calibration analysis showed that the items are highly informative and discriminate between response levels. Regression analysis indicated that emotions are predictors of math achievement. INETAM is solid psychometrically, suitable for measuring academic emotions of adolescents, and can contribute to improve our understanding of their influence on academic achievement in mathematics.