At universities, as in other educational settings, multitasking among students is a widespread phenomenon, leading educational staff to worry about its negative consequences. Using the experience sampling method and adopting a self-determination theory perspective, we examine the situative relationships of autonomous and controlled motivation for studying with multitasking, as well as students' concentration and mood. In total, 138 undergraduate students answered up to six daily questionnaires for 7 days, yielding 1,770 responses during study situations. Results suggest that more autonomous study motivation is associated with less multitasking, while more controlled motivation is associated with more multitasking during studying. Additionally, we found that autonomous motivation is linked to higher levels of concentration and better mood, while controlled motivation is unrelated to concentration or mood. Furthermore, results show that in situations with multitasking in comparison to situations with monotasking concentration was lower, whereas affect did not differ. We used multilevel mediation analyses to test if the situational relationship of motivation and concentration/affect is mediated by multitasking. Mediation analyses did not reveal significant results, although the indirect effect of autonomous motivation on concentration via multitasking was marginally significant. Implications for research on multitasking and self-determination theory are discussed. Also, potential functionalities as well as costs and benefits of multitasking are considered.