Many studies have assessed students’ motivation through surveys, but only a few studies systematically investigated through students’ own words the reasons and aims of their achievement striving. In the present study, we rely on the Goal Complex Perspective (Sommet and Elliot in J Educ Psychol 109(8):1141–1162, 2017. https://doi.org/10.1037/edu0000199; Vansteenkiste et al. in Educ Psychol 49(3):153–174, 2014. https://doi.org/10.1080/00461520.2014.928598) to uncover qualitative differences in achievement motivation. Content analysis of 1303 statements made by 597 Greek secondary students revealed (i) more autonomous motivation than controlled motivation statements, (ii) spontaneously generated statements of mastery-learning and outcome goals but rarely of performance-normative goals, (iii) mastery-learning goals being coupled with both autonomous and controlled motivation, providing ecological support for the conceptualization of mastery goal complexes. Logistic regression analyses verified our classification, as students’ statements were predicted by corresponding types of motivation assessed by close, Likert-type questions. These findings, which align with the Goal Complex Perspective, are discussed in terms of the operational definition of autonomous and controlled motivation and achievement goals.