Building on the multidimensional framework of epistemic cognition proposed by Greene et al. ("Educational Psychologist" 43:142--160, 2008), this study examined beliefs about justification of knowledge claims in science among 65 Norwegian 10th graders. The first research question asked whether beliefs in personal justification, justification by authority, and justification by multiple sources differed in strength among the participants. It was found that the students most strongly believed in justification by authority, followed by justification by multiple sources and personal justification. The second research question asked whether the three types of justification beliefs differentially and uniquely predicted the comprehension of multiple conflicting documents on a science issue. In a multiple regression analysis with multiple-documents comprehension indicated by essay performance as the dependent variable, both personal justification and justification by multiple sources emerged as unique predictors when topic knowledge was controlled for. Specifically, beliefs in personal opinion as a means of justifying knowledge claims in science was negatively related to multiple-documents comprehension, whereas beliefs in justification through corroboration across multiple sources of information were positively related to multiple-documents comprehension. This study provides new evidence about relationships between epistemic beliefs and new literacy competencies needed in an information society, such as integrating across multiple conflicting sources of information; relationships that may also have practical implications.