Background: There has been an increasing interest in negative or 'undermining' motivations for reading. In this study, we aimed to strengthen knowledge on the validity of the distinction between affirming and undermining motivations. First, we examined whether the structure of a questionnaire based on this distinction could be confirmed. Second, we examined the predictive value of undermining motivations for reading comprehension. Third, we studied moderator effects of gender and age. Methods: We administered a reading motivation questionnaire and a reading comprehension test to 324 low-achieving adolescents. The questionnaire included items on affirming and undermining motivations for school and leisure time reading: intrinsic motivation and avoidance, self-efficacy and perceived difficulty. Results: Confirmatory factor analyses supported the assumed structure of the questionnaire. Undermining motivations, particularly perceived difficulty, explained unique variance in reading achievement. Gender and age did not moderate effects of motivational variables. Conclusions: Educators need to be aware of the role of undermining motivations. Future research should examine if interventions can lead to the reduction of such motivations.