The purpose of this study was to determine the best-fitting structural equation model between the freshmen's physics achievement and selected affective characteristics related to physics. These characteristics are students' situational interest in physics, personal interest in physics, aspiring extra activities related to physics, importance of physics, importance of electricity, physics course anxiety, physics test anxiety, physics achievement motivation, student motivation in physics, self-efficacy in physics, self-concept in physics, and locus of control. The researchers developed the affective characteristics questionnaire that consisted of 12 subdimensions, and has 53 items related to these subdimensions. The questionnaire was applied to 890 freshmen physics students at the universities in Ankara. Two models were tested: a unidimensional model and a multidimensional model. However, a third model, which is more similar to the multidimensional model, exhibited the best fit for the freshmen. Moreover, the results revealed that achievement motivation was the most influential affective characteristic on physics achievement. On the other hand, motivation in physics had a negative influence on physics achievement in the model, and the influence of the students' attitudes towards physics was not statistically significant. Thus, one should especially pay attention to the students' achievement motivation in physics if the aim is to increase students' physics achievement. (C) 2007 Wiley Periodicals, Inc.