Understanding the affective factors and beliefs of young students about mathematics is a complex task. This is especially important in the framework of problem solving where these kinds of beliefs are related to various learning processes and influence achievement. In this work, we present the analysis of a questionnaire regarding beliefs about self-competence, self-confidence, mastery goal orientation, effort, difficulty of mathematics, and enjoyment of mathematics applied to Chilean third graders. Exploratory factor analysis leads us to the conclusion that it is possible to measure these kinds of beliefs with a Likert-type questionnaire and that there is an inverse item effect. We tested two confirmatory factor analysis models that allowed us to understand the behavior of inverse items in relation to the mathematics-related affect traits. These models suggest that the inverse item effect is a response style of Chilean children and the affect structure is consistent with the theoretical one considering this effect.